The Day Nui Con Voi mylonitic belt in Southwestern China and Its implications for the early Cenozoic extrusion of Indochina

The Day Nui Con Voi Mylonitic Belt in Southwestern China and Its Implications for the Early

Cenozoic Extrusion of Indochina

by

Erika Swanson

Submitted to the Department of Earth, Atmospheric and Planetary Sciences

in Partial Fulfillment of the Requirements for the Degree of

Bachelor of Science in Earth, Atmospheric and Planetary Sciences

at the Massachusetts Institute of Technology

June, 2007

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The Day Nui Con Voi Mylonitic Belt in Southwestern China and Its Implications for the Early Cenozoic Extrusion of Indochina

by

Erika Swanson Submitted to the

Department of Earth, Atmospheric and Planetary Sciences May 24, 2007

ABSTRACT

The early Cenozoic India-Asia collision resulted in the extrusion of large crustal fragments southeast from the Eastern Himalayan syntaxis, with large shear zones at their boundaries that could have accommodated displacements of hundreds to perhaps a thousand

kilometers. Along the northeastern edge of the Indochina extruded fragment, the belt of mylonitic metamorphic rocks generally referred to as the Ailao Shan/Red River shear zone forms the extrusion boundary. This shear zone actually consists of at least two belts, the Ailao Shan and the Day Nui Con Voi, which are separated by a narrow belt of

unmetamorphosed Triassic sedimentary rocks. In the Chinese extension of the Day Nui Con Voi, the presence of sillimanite and garnet indicates the shear zone formed at

amphibolite grade, and the mylonitic fabric defined by muscovite and biotite indicate left-lateral shearing. Ar/Ar cooling ages indicate the metamorphic rocks reached the cooling

temperature of muscovite and biotite 26.07 t 0.20 to 32.46 0.25 Ma, ages that match

those in the Day Nui Con Voi in north Vietnam. These data come from both the core orthogneiss of the shear zone as well as a narrow carapace of metasedimentary rocks of unknown age. Both rock units form an antiform in southern China that plunges below Triassic sedimentary rocks of South China. These relations show that: 1) the Day Nui Con Voi in China is the direct continuation of the same belt in north Vietnam, 2) the Day Nui Con Voi does not directly connect with the Ailao Shan shear zone, 3) the Day Nui Con Voi shear zone has a structural (?) cover of South China Mesozoic sedimentary rocks, 4) structural relations limit the amount of late stage left-lateral shear on the Indochina

boundary, and 5) the structural relations require a more complex history for the shear zone along the NE boundary of the extruded Indochina crustal fragment than proposed by all earlier workers.

Thesis Supervisor: B. Clark Burchfiel Title: Professor, MIT EAPS Department

Acknowledgments

I would like to thank Clark Burchfiel for his amazing help on every aspect of this

thesis. From taking time to explain the geology and issues as many times as I needed, to his plentiful feedback on my written sections, his help has made this project a very enjoyable experience.

I would also like to thank Malcolm Pringle for his instruction and help with the

Contents

1 Introduction... 5

2 Regional Geology ... 7

2.1 Ailao Shan shear zone... 8

2.2 Day Nui Con Voi shear zone... 9

2.3 Day Nui Con Voi cover rocks in Vietnam...10

3 Rocks of the Day Nui Con Voi and Carapace in Yunnan...12

3.1 M ineralogy ... 12 3.2 Shear Sense... 15 3.3 Geochronology ... 16 4 Interpretation ... 20 4.1 M ineralogy ... 20 4.2 Kinem atics... 21 4.3 Geochronology ... 22

5 Discussion and Conclusions... 23

6 Appendix A ... 34 7 References... 42 List of Figures Figure 1... 26 Figure 2... 27 Figure 3... 28 Figure 4... 29 Figure 5... 30 Figure 6... 31 List of Tables Table I ... 32 Table 2 ... 33

1. INTRODUCTION

About 50 million years ago, the India plate collided with the Eurasian plate. India's continued northward motion deformed the continental crust and built the Himalaya ranges, largely from the upper crust of the Indian crust. The deformed Eurasian

lithosphere formed the Tibetan plateau, the largest area of high elevation and thick crust in the world. The northward motion of India relative to Eurasia since the collision has been about 3200 km, more than half of which must be absorbed within Eurasia. There has been little success in finding this much deformation within Eurasia. Analog models by

Tapponnier et al. (1982) have suggesed that the collision caused extrusion of large regions of lithosphere away from the collision zone towards modem-day Indochina and

southwestern China. Since this model was proposed it has been the topic of much discussion and debate (Zhang et al, 2006 and references therein).

More recently studies (e.g. Wang and Burchfiel, 1997) have suggested that the extrusion involved many different crustal (lithospheric ?) fragments that moved to the SE as a result of the collision, each moving at a different relative velocity and timing. A significant part of the relative motion between these crustal fragments was accommodated

by large strike-slip faults, which may have displacements of several hundred to a thousand

kilometers (Leloup et al, 1995, 2001). Such large displacements would cause significant deformation within the shear zones and surrounding rocks, producing belts of sheared rocks that crop out along the boundaries of the fragments.

There are belts of mylonitic rocks, often referred to as the Ailao Shan/ Red River shear zone, that strike parallel to the proposed shear zone and form the NE boundary of extruded Indochina lithosphere. This early Cenozoic shear zone can be traced from NW Yunnan through north Vietnam to the South China Sea (see Figure 1). These belts of sheared rocks include the Ailao Shan and Day Nui Con Voi mylonitic belts. There is also a younger brittle fault zone, the Red River Fault zone, consisting of several faults that cut through or bound the older broader belt of mylonitic rocks. This right-lateral fault has been considered one of the most important late Tertiary strike-slip faults in the region because of its supposed relation to the eastward extrusion of South China relative to Indochina during the continued indentation of India (Tapponnier et al., 1982, Leloup et al., 1995, 2001).

Many people have worked towards an understanding of these shear zones, especially the Ailao Shan (e.g. Gilley et al., 2003, and references therein) and the

Vietnamese Day Nui Con Voi (see Jolivet et al., 2001, and Gilley et al.,2003). However, the northwestern part of Day Nui Con Voi extends into China where it has not been studied.

This thesis demonstrates that the Chinese part of the Day Nui Con Voi is the direct continuation of this shear zone in Vietnam and has the same argon isotope cooling ages, mineralogy, and kinematics as its Vietnamese counterpart. Equally as important, this thesis also discusses the relation of these rocks to those in the Ailao Shan. The Ailao Shan and the Day Nui Con Voi are not a single continuous belt of rocks, a fact that earlier maps and previous interpretations have ignored (e.g. Leloup et al., 1995, 2001, and Gilley et al.,

2003). This break in the continuity of the mylonitic metamorphic belts and the relation of

the metamorphic belts to their cover rocks have significant implications for the magnitude and mechanism of deformation, and understanding the nature of the early Tertiary

extrusion process.

Understanding deformation of the Day Nui Con Voi and its relation to the Ailao Shan are important not only for the interpretation of the extrusion hypothesis, but also for understanding of the nature of crustal deformation in other continental collision zones where extrusion has been postulated (e.g. Seng6r et al, 1985).

2. REGIONAL GEOLOGY

The Ailao Shan -Red River shear zone has been suggested as the northeastern

boundary of the Indochina fragment as it was extruded to the southeast during early Tertiary time relative to Yangtze/South China continental crust (Figure 1). The southeast movement of Indochina with respect to South China produced a left-lateral sense of motion (Tapponnier et al, 1982). The metamorphic shear zones along this belt show mylonitization with a left-lateral sense of shear.

The geological relations in SE China show the Ailao Shan -Red River shear zone

should be considered as at least two separate structures from distinct deformation events. The Ailao Shan shear zone is an early Tertiary left-lateral shear zone marked by

amphibolite- and greenschist-grade metamorphic rocks, often mylonitized, that strikes

NW-SE and underlies the Ailao Shan range in southern Yunnan, China. The Red River

fault zone is a late Tertiary brittle fault zone that generally parallels the Ailao Shan shear zone on its north side. The Ailao Shan shear zone, the focus of this study, is 400

kilometers long, extending from southwestern China to north Vietnam, and tapers from a pinch out at its NW end up to 20 kilometers wide in the south. Generally regarded as continuous, the Ailao Shan shear zone is not, however, a single connected zone of mylonitic rocks (Burchfiel et al., in press). We show here that there are two major

mylonitic belts, the Ailao Shan to the northwest and the Day Nui Con Voi to the southeast, along with other smaller narrower mylonitic belts in between, a fact shown on all old Chinese maps (Ministry of Geology, 1975). As shown in Figures 2 and 3, the Day Nui Con Voi belt is separated from the Ailao Shan by narrow belts of Cambrian/ Ordovician

(?)

and Triassic sedimentary rocks (Ministry of Geology, 1975).

2.1 Ailao Shan Shear Zone

The Ailao Shan shear zone was an active left-lateral shear zone during late Oligocene and early Miocene time, 32 Ma to 15 Ma (Leloup et al, 1995, Harrison et al

1996, Wang et al 1998). It is composed of metasediments, mostly marbles and

micaschists, and orthogneiss (Leloup et al., 1995). The rocks are metamorphosed to amphibolite grade and are variably mylonitized, with a steeply dipping foliation that

strikes NW-SE, and with a subhorizontal stretching lineation, both of which are parallel to the trend of the shear zone (Leloup et al. 1995). Jolivet et al. (200 1) studied the

continuation of the Ailao Shan in north Vietnam, and reported that the foliation dips near vertically in the eastern part of the shear zone and becomes more gentle to the west, but a left-lateral or top-to-the-northwest sense of shear is found throughout the belt.

The mineralogy of the rocks from the Ailao Shan include sillimanite and garnet, and form an assemblage that is representative of amphibolite (700 C and 6 kbar) and greenschist (480 C and 3 kbar) facies metamorphic conditions (Leloup and Kienast, 1993 and Tran Ngoc Nam et al, 1998). Kyanite has not been found within the Ailao Shan shear

zone. "0Ar/39Ar cooling ages for monazite within the Ailao Shan shear zone have generally

ranged between 21-34 Ma, but older ages, up to 74 Ma are also reported (Gilley et al,

2003).

2.2 Day Nui Con Voi shear zone

The Day Nui Con Voi is a belt of mylonitic metamorphic rocks that lies mostly in Vietnam, but continues into China (see figure 3). It is bounded by brittle faults of the younger Red River fault zone. The mylonitic rocks belonging to the older, mylonitic shear zone consist of ortho- and paragneisses with some leucogranites (Jolivet, 2001; Tran Ngoc

Nam et al, 1998). The Day Nui Con Voi contains gamet-sillimanite micaschists, gneisses, and minor amphibolites, marbles, pegmatites and migmatites (Jolivet et al., 2001).

At the northern end of this belt in Vietnam, foliations define an open to tight antiform that plunges gently N 140E. The antiform is parallel to both the shear zone and its

lineation (Jolivet et al, 2001). The metamorphic rocks show a left-lateral sense of shear which is strongest along the limbs of the antiform and more symmetrical near its core

(Jolivet et al, 2001). The mylonitic foliation is parallel to the axial plane of the antiform suggesting they are related to the same deformation.

Jolivet et al. (2001) reported two distinct metamorphic events from the Day Nui Con Voi, one greenschist facies, and one amphibolite facies. The higher temperature deformation corresponds with the subhorizontal foliation in the core of the Day Nui Con Voi antiform, whereas the lower temperature deformation is related to the steeply dipping

left-lateral deformation. Gilley et al (2003) reported 40Ar/39Ar cooling ages that ranged

from 220-44 Ma for monazite within the Day Nui Con Voi. They interpret the

amphibolite facies metamorphism to have occurred during the early Mesozoic Indosinian orogeny, with only partial resetting of the monazite compositions. Argon cooling ages of

21-25 Ma have also been found in the Day Nui Con Voi (Maluski et al, 2001).

Microscopically the left-lateral shearing is well developed, with shear bands and pressure shadows showing evidence of shear deformation after the crystallization of

sillimanite, but contemporaneous with the crystallization of micas. Both sillimanite and garnet are fractured along the shear bands (Jolivet et al, 200 1).

2.3 Day Nui Con Voi cover rocks in Vietnam

When traced to the NW into southern China, the Day Nui Con Voi mylonitic gneiss ends in an antiform that plunges NW below some Paleozoic, but mostly Triassic metasedimentary and sedimentary rocks. It is not known whether this cover is

depositional or tectonic (Burchfiel et al, in press). The Triassic rocks are dated by fossils and comprise a section that is characteristic for South China. Between the mylonitic gneiss in the core of the Day Nui Con Voi and well dated Triassic sedimentary rocks are a narrow belt of metamorphosed and mylonitic metasedimentary rocks that forms a carapace for the core gneiss. The age of these mainly metaclastic rocks are shown as lower Triassic on Chinese maps, but their age and relations to dated Triassic rocks is not known.

Samples of these rocks and their geochronology are described below.

The mylonitic rocks of the Day Nui Con Voi and their carapace of

metasedimentary rocks are separated from mylonitic rocks of the Ailao Shan shear zone

by a narrow belt of Upper Triassic rocks. These rocks are unmetamorphosed and contain

coal deposits and fossils characteristic for Upper Triassic strata of South China. These unmetamorphosed strata along the south flank of the Day Nui Con Voi are traced to the Vietnam border where they are continued by a belt of sedimentary and metasedimentary rocks shown on Vietnamese maps as Cambrian. Within Vietnam the unmetamorphic strata of Upper Triassic character ends and the belt continues as meta sedimentary schist of

assigned Cambrian age, but the age of these rocks remains suspect. They are lithologically similar to the metasedimentary rocks that form the carapace of the core gneiss in Yunnan.

3. ROCKS OF THE DAY NUI CON VOI AND CARAPACE IN YUNNAN

Rock samples from the core gneiss and their carapace were collected by B. C. Burchfiel during the summer of 2006. The location of samples is shown in Figure 3. Many of these rocks were selected for thin section analysis. Mineralogy, shear sense and

40_Ar/39_{Ar analysis of these samples is reported below for comparison with data published}

for the Ailao Shan and Day Nui Con Voi shear zones.

3.1 Mineralogy

The thin sections were cut perpendicular to the foliation and parallel to the lineation, in order to show the greatest asymmetries for identifying the shear sense. They were observed in both plane-polarized light and under crossed-polars. Digital

photographs were taken with a 5 Megapixel camera light mounted on a Zeiss Discovery

Core metamorphic rocks

Most of these samples (SPBs, Hs, Hes, and RR-IAB and RR-2AB) are mylonitic orthogneisses of granitic protolith. They generally have prominent feldspars, highly sheared quartz, and varying quantities of muscovite and biotite, and some samples contain sillimanite and/or garnet. A list of mineralogy by sample can be found in Table 1.

The foliation of these samples is usually well defined by micas, although in some samples, especially those with less muscovite and biotite (especially RR-1AB, SPB-1, SPB-2 and SPB-6) the foliation is less well defined. However, in all samples, the mica have a preferred orientation that defines the s-fabric.

In addition to muscovite and biotite, sillimanite is present in many of the samples (SPB-4,

SPB-5, He-1, and He-2) and lies within the s-surfaces. Quartz is highly recrystallized and

forms flattened and elongated grains parallel to the fabric. In many samples, these s-fabric features showed further deformation. In a few samples, a clearly defined C-s-fabric cross-cut the S-fabric. In He-3, this C-fabric is orientated parallel to grains of chlorite and fine-grained biotite. In SPB-4, it is parallel to muscovite and fine-grained alteration minerals. In all other cases, the C-fabric was present but not well-defined.

In all samples, even those without clear C-fabrics, the appearance of the S-fabric was usually undulating, forming variably well developed mica fish, as shown in Figure 4 a and b. Sillimanite is often fractured perpendicular to its length and perpendicular to the

s-fabric. The orientation of the garnet fractures was not always perpendicular to th s-fabric, but often within 250 of it.

Carapace rocks

The samples that make up the carapace surrounding the orthogneissic metamorphic core of the shear zones (Gs and RRs) generally contain more micas. These rocks are

paragneisses, usually schists. Some of them (RR-1, RR-4, and RR-5) have been strongly

altered, making mineral identification difficult. In addition to muscovite and biotite, these samples contain quartz, a few contain calcite or chlorite, and many contain sillimanite and/or garnet. A list of mineralogy by sample can be found in Table 1.

The foliation in these samples is also well defined by layers of muscovite and biotite. Some of the RR samples are heavily altered, and it is difficult to determine the original mineralogy of the foliation. Between the patterns of fine-grained masses and the sheared and flattened quartz, a foliation is present. Some of the sillimanite is parallel to the s-fabric, but it is also found in feathery masses. In the samples with calcite (G-2, RR-1), the calcite usually occurs parallel to the S-fabric, but in RR-1 has also crystallized in fractures in throughout the rock.

All of the samples show further deformation superimposed on the S-fabric, but it

does not always form a clear foliation. Where it does, if forms a well developed C-surface that deforms the S-surface micas forming a well-developed S/C fabric (see Figure 4 c and

d). Of the handful of samples that show a defined C-fabric, that of RR-4 MS is

particularly clear, marked by biotite and some recrystallized quartz, as shown in Figure 4

d. In one sample (Dimi- 1) there are well developed rolled garnets suggestive of a

right-lateral shear sense (see below).

3.2 Shear sense

Nearly all of the samples with a clearly observed sense of shear were left-lateral. The samples either had a clear left-lateral sense of shear or one that was ambiguous. Evidence included mica fish, pressure shadows/ wings on porphyroclasts, and S-C fabrics. Figure 4 shows photographs of these features.

Sample Dimi- 1 showed rotated garnet grains (Figure 4-f), with a pattern of quartz inclusions that indicates the gamet grains rolled during crystallization. The pattern implies a right-lateral sense of motion, but is not definitive, as the quartz inclusions cannot be related to the fabric. There is insufficient asymmetry in the S-fabric of this sample to determine an unambiguous shear sense.

3.3 Geochronology

Ten of the samples with the best-looking micas were selected for Ar/Ar analysis. To isolate suitable mica crystals for Ar/Ar geochronology, the samples were crushed using ajaw crusher, and sorted by dry sieve into >1.0mm, 500-1000 p m, 250-500 p m, and <250 p m portions. Rock fragments that were >1.0 mm in size were passed through the jaw crusher repeatedly, until most of the sample was less than 1.0 mm in size. Some samples (SPB-1, SPB-2, SPB-6, and G-3) also required the use of the disc mill in order to crush most of the sample to <1 mm. After crushing, a 125 p m sieve was added to the bottom of the sieve stack, the samples were washed in water, and then dried under a heat lamp.

The ideally sized separates were then sorted for micas, both muscovite and biotite if available and sufficiently clean, as determined by observation under a binocular

microscope. The largest size fraction that contained fresh, isolated crystals was used. For most samples, the ideal size was the 500-1000 p m fraction, but samples for G-1, G-3, and

SPB-6, it was necessary to use the 250-500 p m fraction.

Samples H-2a, G-1, G-3, SPB-6, and He-I were sorted with a Franz magnetic separator to concentrate the biotite. Samples RR-4MS, G-1, G-3, SPB-1, SPB-2, RR-2A, and He-3 were sorted for micas by using a statically charged plastic sheet to preferentially

attract the flatter minerals with higher surface area/volume ratio, while other minerals fell onto a paper below. The statically trapped minerals were scraped off the plastic sheet,

onto a paper sheet, and poured into containers. This method was particularly useful for concentrating muscovite.

After separation, the minerals were cleaned with alcohol in a 40 kHz ultrasonic bath sonicated for 60 minutes, poured on to a 125 p m sieve, and air dried. The dried minerals were then hand-picked, selected for primary by size and freshness. The best -50 crystals were reserved for single crystal analysis, while other suitable crystals were picked for bulk analysis. The size and mineral of each sample can be found in Table 1.

The mica crystals for single grain work were loaded into commercial aluminum foil packets, whereas micas selected for bulk analysis were loaded into 99.99+% copper foil packets. Both sets of foil packets were loaded into 8 mm diameter, 75 mm long, fused-silica vials, and irradiated without Cd-shielding for 8 hours at a power level of 3 MW in the C5 position of McMaster University Nuclear Reactor in Canada.

After irradiation, micas for single crystal laser-fusion analyses were unloaded from the Al foil packets, further hand picked into individual 2 mm diameter, 3 mm deep holes on a 100-hole Cu planchette, loaded into a UHV sample chamber with a glass viewport, and baked out at c. 320' C for 8 hours before cooling to room temperature. For the actual Ar/Ar analysis, each crystal was heated with a c. 1.5 mm diameter 810 nm diode laser beam, ramping from 0 to 15 W over 30 seconds and then held at 15 W for 15 seconds. The ramped heating and defocused laser beam minimizes the amount of mica jumping during heating, a problem likely encountered significantly more than is typically reported for

typically begins to fuse between 1 and 2 W, muscovite by 4 W, and complete fusion of both types of micas to a refractory glass ball nearly always occurs. After fusion, the released gases were purified for 10 minutes with a SAES glass-encased 10 1/sec StiOl getter operated at c. 650 deg, and a second, all metal, SAES 50 /sec Sti01 getter operated c. 350' C. The purified gases were then admitted to an MAP 215-50 mass spectrometer

for Ar isotopic analysis using a Balzers 217 electronic multiplier operated at a gain of about 10,000.

Mica crystals were usually not degassed with the laser beam before analysis as is

sometimes the practice with CO2 laser-based mica fusion studies. Our experience is that

the need for such a cleaning step is significantly reduced with the higher bake-out temperatures possible with the glass viewports that are possible with a diode laser but not

the special mid-infrared viewports necessary with CO2 lasers (i.e., >3000 C for glass

viewports versus < 150' C for typical ZnS windows). However, we did try degassing a

small subset of samples with a 0.5 or 0.75 W heating laser step. In general, the degassing steps did seem to make a margin improvement in the radiogenic 40Ar content of some of the subsequent fusion steps, but did not seem to make enough of an improvement to warrant the application of a cleaning step to the whole mica population.

Micas for bulk analyses were heated using a low blank double vacuum furnace controlled by a C-type thermocouple and Eurotherm-based controller using a 2 min ramp time and 10 minute heating step. The released gases were cleaned during the heating step and by an extra 10 minute exposure to an SAES 50 1/sec Stl0l getter operated c. 400' C. This was to get the reactive gases as well as 1 gram of degassed 13X molecular sieve to

help absorb water and hydrocarbons. The purified gas was then analyzed to the same

MAP215-50 mass spectrometer used for the single crystal laser-fusion measurements.

The conversion efficiency of 39K to 39Ar was monitored using laser analysis of

co-irradiated sanidine from the Taylor Creek rhyolite (TCR-2a); all ages reported here are relative to an age of 28.34 Ma for TCR-2a (Renne et al., 1998). Corrections for neutron-induced interferences, previously determined for this reactor facility using Fe-doped kalsilite glass and optical CaF2, were 0.029 for 'Ar/

9

ArK, 0.000672 for 39Ar/37Arca, and

0.00028 for 36Ar/37Arca. All data reduction and age calculations were made using the

program ArArCalc version 2.2 (Koppers, 2002). All errors are reported as 2 standard deviation (2 a ) of analytical precision.

The 40Ar/39Ar data yield ages that ranged from 26.07 0.20 to 32.46 0.25 Ma.

The results are summarized in Figure 5 and Table 2, and complete data tables may be found in Appendix A. These ages are consistent with ages reported from the Day Nui Con Voi in Vietnam by Jolivet et al. (2001) and Gilley et al. (2003).

4. INTERPRETATION

The mylonitic rocks present in Yunnan (SE China), are the direct continuation of the Day Nui Con Voi mylonitic belt in Vietnam and have the same geological

characteristics. They have the same mylonitic fabric, orthogneissic core, kinematic sense of shear, grade of metamorphism, and Ar/Ar cooling ages.

These rocks also show similar characteristics to those in the Ailao Shan shear zone, suggesting a similar metamorphic and deformational history. However, the Ailao Shan and the Day Nui Con Voi are separated by a narrow belt of unmetamorphosed Triassic rocks and the Day Nui Con Voi mylonites have a cover of Triassic rocks of South China facies.

4.1 Mineralogy

The presence of garnet and sillimanite indicates that these rocks have undergone relatively high grade metamorphism. It also indicates that the Chinese part of the Day Nui Con Voi experienced the same pressure-temperature conditions as those in Vietnam. Also notable is the absence of kyanite, indicating relatively low-pressure conditions in both locations.

The current state of fracturing and alteration of the garnet and sillimanite grains indicate further deformation after the crystallization of these minerals. This supports the interpretation by Leloup and Kienast (1993) that the high-grade metamorphism was followed by a retrograde metamorphism during mylonitization.

4.2 Kinematics

Most of the rocks sampled showed a left-lateral sense of shear. This confirms that the rocks in the Chinese part of the Day Nui Con Voi experienced similar stress

orientations as those in the Vietnamese Day Nui Con Voi, and also those in the Ailao Shan.

Many of the rocks sampled, especially samples G-1 through G-4, had significant overprinting of younger deformation. Other samples have undergone alterations that make the grain boundaries and fabrics less distinct. Due to these factors, the shear sense was ambiguous in many of the samples and more detailed sampling is necessary to resolve the detailed kinematics of these rocks.

4.3 Geochronology

The Ar/Ar radiometric analysis in the Day Nui Con Voi mylonitic rocks in China

yielded a range of cooling ages from 26.07 0.20 to 32.46 0.25 Ma. They support the

interpretation that the mylonitic belt of rocks in China are not only the direct continuation of lithologically similar rocks in Vietnam, but have a similar kinematic and cooling history.

The samples that were heated in steps were found to have little difference in measured argon levels, indicating relatively low levels of contamination. These ages are consistent with ages reported from the Day Nui Con Voi in Vietnam by Jolivet et al. (2001) and Gilley et al. (2003). They support the interpretation that the mylonitic belt of rocks in China are not only the direct lithological continuation of similar rocks in Vietnam, but have a similar kinematic and cooling history.

There is little variation seen in the ages, although this does not necessarily

represent a lack of variation in age of the grains. Our methods selected for the clearest and least-altered grains, which may have prevented potentially older grains from being

sampled. It is important for further radiometric analysis to see if the evidence for an older metamorphic event present in Vietnam is also present in the rocks from China.

5. DISCUSSION AND CONCLUSIONS

The rocks from the Chinese Day Nui Con Voi were metamorphosed at amphibolite grade during left-lateral shearing at depths of up to 20 kilometers. During subsequent exhumation, the rocks experienced retrograde metamorphism and mylonitization of the higher grade original fabric. During Oligocene to Miocene time, exhumation resulted in conditions sufficiently cool to be below the closure temperature for Argon in biotite and muscovite. This history matches that of the Vietnamese Day Nui Con Voi and Ailao Shan.

The similarity in rock types and histories suggest that the Ailao Shan and the Day Nui Con Voi were once part of the same mylonitic metamorphic belt, but they are currently separated by young faults and unmetamorphosed Triassic sedimentary rocks, indicating subsequent structural activity led to their present-day separation. This

difference in structural position between the two mylonitic belts is further complicated by the fact that the mylonitic core and carapace rocks of the Day Nui Con Voi form an antiformal nose that plunges NW in China below fossiliferous Triassic strata. This places an unmetamorphosed cover sequence of South China strata above the mylonitic rocks.

Although Chinese maps show these carapace rocks are in depositional contact with the gneissic core rocks, the contact has not been observed in the field. Unless the contact does show an original unconformable relation, the common grade of metamorphism and deformation does not support a Triassic age. If they are in fault contact, the fault must be pre-metamorphic in age. Regional relations now suggest the carapace rocks are in fault

contact with the overlying fossil-bearing Triassic sedimentary sequence. Further fieldwork is needed to determine the relation of these cover rocks to those in the metamorphic shear zones.

The relation between the mylonitic core rocks and their cover rocks has important implications on the amount of displacement that could have been accommodated along these shear zones, but there has been disagreement about such a relationship.

While the Day Nui Con Voi rocks are covered by unmetamorphosed Triassic rocks, nowhere do the Ailao Shan mylonitic rocks have a definite cover above them.

Everywhere else in South China, the stratigraphic sequence is continuous from at least Devonian, and in most places from Cambrian, to Upper Triassic. Thus the cover rocks for the Day Nui Con Voi are interpreted to be in fault contact with the metamorphosed

carapace rocks. This fault must be folded in the antiform. We hypothesize that this fault was a low-angle fault with probable normal displacement to cut out the lower part of the

South China section.

The presence of a structural (?) or stratigraphic (?) South China cover for the Day Nui Con Voi mylonitic belt suggests that the magnitude of left-lateral displacement on the Day Nui Con Voi, and perhaps the entire early Cenozoic shear zone may need to be reevaluated.

Our present interpretation of the relations in South China suggest that the mylonitic rocks were formed from basement, and possibly some Paleozoic cover rocks of South China. During metamorphism and mylonitization, left-lateral shearing occurred in a

transpressional setting so that the more ductile rocks in the shear zone had a component of upward extrusion from beneath the cover rocks forming an apparent normal fault above

the mylonitic rocks, as shown in Figure 6 (See also Wang and Burchfiel, 1997). This hypothesis will be a focus of further field study to determine the relations between the metamorphic belts and the carapace rocks to the unmetamorphosed sedimentary rocks in

surrounding area.

The relation of these metamorphic belts to each other and to the surrounding rocks is important for understanding the kinematics of the extrusion of the Indochina tectonic fragment during the India-Asia collision. This area needs further study in order to

understand more completely the history of these metamorphic zones and their implications for continental collisions in general.

bNN

China

Figure 3

1I

India

Indochina

Figure 1: A tectonic map showing the generalized motion of tectonic fragments in Southeast Asia. According to the model proposed by Tapponnier et al. (1982), the Indochina fragment moved towards the southeast first, causing left-lateral shear along the fragment boundary, thought to be the Ailao Shan-Day Nui Con Voi shear zones. As

that movement slowed, the eastward extrusion of China increased, causing the relative

motion along the same shear zone to become right-lateral. Modified from Tapponnier et al., 1982.

0 100 km

Mesozoic

Upper Paleozoic

]

Lower Paleozoic

Ailao Shan

Precambrian

C

Plutons

Day Nui Con Voi

Neogene

Melange

Figure 2: Geologic map of the Ailao Shan and Day Nui Con Voi. The metamorphic

belts of mylonitic granites are shown to be discontinuous, separated by a zone of

younger, unmetamorphosed rocks. The location of Figure 3 is shown by the black

outline.

0 10km TRIASSIC UNITS PERMIAN LOWER PALEOZOIC China MYLONITiC METAMORPHIC Vietnam ROCKS LATE PROTEROZOIC border limit of metamorphism

Figure 3: Geologic map showing the locations of the samples studied. Those within the

pink zones are considered core rocks, while those outside, but within the dashed line are considered caraoace rocks. The Day Nui Con Voi and the Ailao Shan contain both unmetamorphosed Triassic rocks and metamorphosed carapace rocks between the mylonitized granitic cores. The northernmost end of the Day Nui Con Voi plunges beneath metamorphosed carapace and unmetamorphosed Triassic rocks.

C

qa

~

4

m~

~

Figure 4: left-lateral shear sense can be observed in samples a-e, and sample f has ambiguous shear sense. a) sample SPB-4 at 1Sx magnification, b) He-3 at 25x magnification, c) RR-2 at 8x magnification, d) RR-4MS at 15x magnification, e) SPB-2 at 8x magnification, f) Dimi-1 at 25x magnification

N-V

a

_{_7}

AV

gal-7 -IML1 -S

Mr-- 4W me- _*

30 -G-1 Msc: 31.9 0.2 Ma G-3 Msc: 31.1 0.2 Ma 25 25 RR-4 Msc: 26.1 0.2 Ma =ll 20 .0 15 10 5

0

20 25 30 35 40

Mica Ar/Ar Age (Ma)

40 _

Core

- H-2a Msc: 32.5 0.3 Ma 35 - H-2a Bio: 31.1 0.6 Ma SPB-1 Msc: 29.5 0.1 Ma 30

ftSPB-2

Msc: 29.5 0.1 Ma SPB-6 Bio: 26.9 0.1 Ma 25 10 0-5

0

20 25 30 35 40

Mica Ar/Ar Age (Ma)

Figure 5: Plots of the distribution of the Ar/Ar cooling ages of individual grains, sorted

Tr ul Linchanq P -Tr Tr val Linchang granites N P-Trr A ro ng Smao ~o c ut 01Z older normal

Figure 6: Proposed reconstruction resulting in current separation of the Ailao Shan and

Day Nui Con Voi. Initially, the shearing mylonitized a zone along a non-vertical fault,

with Mesozoic sediments above the sheared rocks. After a fabric had developed but

before the end of the deformation, the shear zone is broken up, with slivers of

unmetamorphosed Triassic sediments separating the two belts.

Lanping-Siman

I

N

Table 1

Sample

mineral and size used

number for Ar/Ar dating

Minerals found

Carapace

Dimi-1 biotite, muscovite, chlorite, quartz, garnet (with quartz inclusions)

G-1 msc 250-500 biotite, muscovite, feldspar,quartz, garnet, sillimanite

G-2 biotite, calcite, chlorite,

G-3 msc 250-500 biotite, muscovite, feldspar, quartz, garnet

G-4 muscovite, feldspar, quartz, garnet

RR-1 inuscovite, calcite, quartz

RR-2 (msc 500-1000) biotite, muscovite, quartz, garnet, sillimanite

R-4 LG iotite, muscovite, feldspar, quartz, garnet

RR-4 MS msc 500-1000 Lbiotite, muscovite, feldspar, quartz, garnet, sillimanite

RR-5

miuscovite,

chlorite, feldspar, quartz, sillimanite Core

HI-i biotite, muscovite, feldspar, quartz, garnet,

H-2

biotite

msc 500-1000 biotite, muscovite, quartz, garnet

He-1 (biotite 500-1000) biotite, feldspar, quartz, garnet, sillimanite

He-2 biotite, quartz, garnet, sillimanite

He-3 (msc 500-1000) biotite, muscovite, quartz, garnet

He-4 biotite, muscovite, feldspar, quartz

RR-1 AB biotite, chlorite, feldspar, quartz

PR-2 AB biotite, chlorite, feldspar, quartz

SPB-1 msc 500-1000 biotite with chloritic alteration, muscovite, feldspar, quartz

SPB-2 ms 500-1000 biotite, muscovite, feldspar, quartz

SPB-4 biotite, muscovite, quartz, garnet, sillimanite

SPB-5

biotite, feldspar, quartz, sillimanite

SPB-6 biotite 250-500 biotite, feldspar, quartz

msc=muscovite

32

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Table 2

Single grain analysis

Core Information en Analysis Sami pie Sample Maerial Sample Sample Maeial Sample Mania H-2a B-31 Biotite H-2a B-32 MuscoviA& SP8-1 B-36 Muscovile SPB-2 8-37 Muscovile SPB-68-38 iotile Carapace Information on Analysis Sample RR-4 MS B-33 Mateial Muscov"Il Sample G-1-34 Maeral Muscovile Sam pie G-3 8-35 Material Muscovile

Bulk analysis

Core

Information on Analysis He-i A-7 Batite 250-500u He-3 A-8 Msc 250-500u RR-2A A-9 Msc 250-500u SP84i A-11 Msc0.541 m m SPB46A-13 Biotite 250-500u SPS-2A-12 MSC 500-1000

Results 40M99k*2v Aget 2 K/Cat2Y

"Eufer Mw &9387 31.03 209.37 67.748t19.930 1.90% t 195% 25 of25 "EnraMon 6.4711t052 3246 5159 100.00 0348+1110 e176% t0.78% 320f25 0.0223 29.50.12 100.00 64 4 "Er o Mewn 63100 t.23 295 2 42 10.0 6.M4 0.408 2.35% t 0.40% 25 WgMedMan 62979 0.0164 2 0.10 1.9 91. 34059 0.645 10.26% t0.33% 22 of25 W1*0gMedMman 5.7313 t U173 26.870.10 1.02 81.86 2451 0.993 0.38% 0.36% 25

Results 40(r>391k)*20 Agetm 39Artk) K/Ca 2a

(Ma M.

"Error Mean 5.555 26.07 i0.20 100.00 1.709t0.417

0.73% i 0.75% 25

"EfarsMan 68320.0314 31.5 r.0.16 9 10000 22598 0596

0.46% t 050% 25

fErrau Mean 6.6514 t639 31.06 t 016 970 94.43 39.931 10291

0A.46% r050% 23of25

Results 40(r39(k) 2( Age MSWD 39A K/Ca 2a

(Ma (%n) Weighted Plateau WeliltedPlateau ErrorPlateau 5.1406 10.0168 710.33% 5.6751 t002 1 0.22% 24.13 0.11 20.41% 26.63 0.10 t0.37% 5.7012 26.76 0.34% 0.45% WeightediPlateau 6.4299 3 30.16 10.18% 0.35% lrrarPlateau Enw Platau 5.8204 27.28 10.54% 0.62% 6.3653 0.04W4 29.87 10.21 a063% t0.70% 1.91 91.28 0.6 81.35 0.66 _{15 d21} 3.6 94.73 3.16 ₁ 20 17 d20 0.90 1529 2.40 64.30 8 4.08 62.86 10af20 105.011129.136 3.091 3.113 9.521 5.017 13.992 14.954 70.640 24.983 12.253 2.205

Appendix A

H-2a biotite

Incre mental 36Ar(a) 37Ar(ca) 38Ar(ci) 39Ar(k) 4OAr(r) Age 4) (r) 39Ar(k) K/Ca 2a

7B006@13 z13 1 .00361 .00096 .01078 .70568 4.40393 27.93 0.19 80.20 7.64 361.283 157.133 7Z006@02 z2 / .00268 .00067 .00698 .45470 2.84036 27.96 0.21 77.93 4.92 330.639 181.713 7B006@11 11 / .00277 .00089 .00799 .51071 3.29331 28.85 0.22 79.82 5.53 282.022 99.713 7A005@05 a5 / .00249 .00063 .00551 .34881 227615 29.19 0.29 75.28 3.77 269.445 129.430 7Z006@05 z5 / .00032 .00034 .00362 .23653 1.57636 29.81 0.21 93.89 2.56 342.670 454.186 7A005@03 a3 / .00110 .00025 .00470 .28556 1.92713 30.18 0.29 85.28 3.09 563.332 610.067 78006@14 z14 / .00116 .00103 .00766 .49828 3.36335 30.19 0.17 90.42 5.39 235.906 78.643 7Z006006 z6 / .00184 .00066 .00531 .31966 2.16754 30.33 0.23 79.68 3.46 236.507 155.324 7Z006@08 z8 / .00073 .00024 .00399 .25971 1.76673 30.42 0.21 88.80 281 521.048 722.438 7A005@09 a9 / .00159 .00054 .00446 .27580 1.88923 30.63 0.25 79.76 298 248.542 160.297 7A005@04 a4 / .00086 .00081 .00380 .24849 1.72418 31.03 0.24 86.86 269 150.084 57.751 7B006@12 z12 / .00029 .00078 .00416 .27030 1.87613 31.04 0.19 95.28 2.93 169.750 70.246 7Z006003 z3 / .00083 .00066 .00810 .52983 3.68208 31.07 0.15 93.37 5.73 393.141 186.561 7A005@01 al / .00208 .00043 .00662 .44277 3.10313 31.34 0.21 83.15 4.79 505.081 428.781 7Z006@07 z7 / .00091 .00026 .00600 .38281 2.68712 31.38 0.19 90.61 4.14 724.800 982.451 7A005@02 a2 / .00122 .00047 .00636 .41027 2.87992 31.38 0.19 88.54 4.44 428.001 277.411 7Z006@10 z10 / .00009 .00120 .00169 .10887 .76520 31.43 0.31 96.26 1.18 44.397 20.672 7A005@10 a1O / .00100 .00001 .00347 .21923 1.54219 31.45 0.36 83.57 237 9193.846### 7Z006@09 z9 / .00071 .00394 .00659 .43608 3.09806 31.76 0.14 93.26 4.72 54238 7.107 7Z006@01 z1 / .00108 .00146 .00652 .42379 3.02369 31.90 0.19 90.15 4.59 141.816 46.921 7A005008 a8 / .00101 .00082 .00615 .38857 2.85407 32.83 0.22 90.16 4.21 232.614 86.265 7Z006@04 z4 / .00028 .00066 .00449 .28122 2.06738 32.86 0.19 95.77 3.04 208.516 108.912 7A005@06 a6 / .00116 .00034 .00414 .25589 1.89566 33.11 0.31 84.38 277 373.953 377.617 7B006@15 z15 / .00093 .00154 .01014 .68289 5.10324 33.39 0.17 94.52 7.39 217.604 42.047 7A005@07 a7 / .00105 .00069 .00425 .26349 1.97836 33.55 0.26 86.09 285 187.803 131.603

I

.03180 .02029 .14350 9.23993 63.78452

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Information on Analysis Sample H-2a B-31 Material Biottle Location Erikas Single xl micas

Analyst msp Project Irradiation J-value Standard Malcolm's Mica's cl160-0 28.34

Results 40(r)/39(k) 20 Age 2 39Ar(k) K/Ca 2a

(Ma}) . (% nl

Error Plateau 6.9387 31.03 209.37 10000 67.748 19.930

1.96% 1.95 % 25

External E rro r 0.60 2.06 Statistical T ratio Analytical Error 0.60 14.4695 ErrorMagnification

TotalFusionAge 6.9031.0.0099 370.04 25 223.127 20.788 6 0.14% 0.14% External Error 0.04 Analytical Error 0.04

34

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~~~~~~~

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~

'88

RR-4 muscovite

Incremental 36Ar(a) 37Ar(ca) 38Ar(d) 39Ar(k) 4OAr(r) Age t(7 4OAr(r) 39Ar(k) K/Ca 20

Heatina (Mal (%) (%) 7Z006@39 39.00 W / .00027 .00029 .00044 .22944 1.25059 25.4510.17 93.61 263 389.478 530.169 7Z006@34 34.00 W / .00107 .00093 .00158 .83168 4.55610 25.57 0.12 93.03 9.54 437.784 205.838 7Z006@33 33.00 W / .00077 .00042 .00081 .44829 2.45708 25.59 0.15 91.10 5.14 524.673 453.143 7Z006@35 35.00 W / .00053 .00017 .00079 .40259 2.20912 25.62 0.12 92.94 4.62 1129.116 2571.483 7A005022 22.00 W / .00059 .00034 .00030 .16534 .91024 25.70 0.28 83.62 1.90 239.301 302460 7A005@11 11.00 W / .00047 .00042 .00061 .31865 1.75611 25.73 0.17 92.26 3.66 368.544 391.874 7A005@17 17.00 W / .00058 .00015 .00058 .34971 1.92774 25.73 0.18 91.41 4.01 1124.812 3217.060 7A005@18 18.00 W / .00013 .00004 .00021 .11448 .63205 25.77 0.34 93.82 1.31 1397.708 13029.719 7A005@13 13.00 W / .00066 .00029 .00056 .31828 1.75812 25.78 0.21 89.60 3.65 533.900 784.754 7A005@20 20.00 W / .00031 .00021 .00043 .22491 1.24591 25.86 0.19 92.68 2.58 524.263 1097.033 7A005@15 15.00 W / .00037 .00026 .00028 .17130 .95106 25.92 0.27 89.16 1.96 318.692 550.528 7Z006@40 40.00 W / .00089 .00038 .00062 .35230 1.96396 26.02 0.21 87.84 4.04 449.718 499.115 7A005@21 21.00W / .00055 .00073 .00047 .28697 1.60173 26.05 0.16 90.31 3.29 193.889 115.086 7Z006@32 32.00 W / .00123 .00077 .00168 .87411 4.88295 26.07 0.12 92.63 10.03 553.477 272.507 7A005@14 14.00 W / .00038 .00017 .00033 .18185 1.01977 26.17 0.25 89.67 209 529.357 11332.818 7Z006@38 38.00 W / .00029 .00081 .00054 .24877 1.39642 26.20 0.19 93.73 285 150.586 66.831 7A005@24 24.00 W / .00065 .00034 .00046 .23328 1.31060 26.22 0.20 86.74 268 332.765 403.604 7A005@25 25.00 W / .00075 .00013 .00038 .20525 1.16625 26.52 0.27 83.69 2.35 753.17812064.344 7Z006@31 31.00 W / .00146 .00066 .00156 .81069 4.61422 26.56 0.13 91.01 9.30 599.623 380.018 7A005@19 19.00 W / .00068 .00026 .00051 .29681 1.69045 26.58 0.21 89.00 3.40 558.224 963.642 7A005@12 1200 W / .00037 .00005 .00054 .30768 1.75462 26.611 0.20 93.61 3.53 2914.570 28493.835 7Z006@37 37.00 W / .00077 .00052 .00076 .40769 232650 26.63 0.15 90.63 4.68 385.936 254.034 7A005@16 16.00 W / .00085 .00000 .00048 .25785 1.49153 26.99 0.21 85.24 296 38.886 0.000 7Z006@36 36.00 W / .00131 .00060 .00085 .40801 2.36515 27.05 0.22 85.59 4.68 330.524 194.905 7A005@23 23.00 W / .00092 .00034 .00050 .27192 1.58110 27.13 0.27 84.96 3.12 387.878 466.891 Z .01685 .00931 .01627 8.71786 48.81937 Information on Analvsis Sample RR-4 MS B-33 Material Muscovite Locaion Erikas Single xI mica

Analyst Project Irradiaion J-value Standard msp 2007 0 28.34

Results 40(r)/39(k) 2G Age 2 39Ar(k) K/Ca 2a

(Ma) C' (%.n)

Error Plateau 5.5855 0.0407 2607 0.20 100.00 1.709 0.417

0.73% 0.75% 25

External Error 0.20 206 Stastical T ratio Analytical Error 0.19 5.3264 Error Magnificaion

Total Fusion Age 5.5999 26.14 25 458.743 101.544

t 0. 14% 0.24%25 48731054

External Error 0.06 Analytical Error 0.04

G-1

muscovite

Incremental 36Ar(a) 37Ar(ca) 38Ar(cl) 39Ar(k) 40Ar(r) Age 20 4OAr(r) 39Ar(k) K/Cat2

Heatina (Ma) (%) (%) 7Z006@45 45.00 W / .00028 .00045 .00053 .24195 1.61426 31.13 0.21 94.81 4.45 264.187 225.950 7A005@32 32.00 W / .00025 .00023 .00026 .13073 .87489 31.23 0.32 91.98 2.40 275.182 392.599 7A005@27 27.00 W / .00024 .00041 .00037 .21392 1.43292 31.26 0.21 94.95 3.93 254.916 185.180 7A005@38 38.00 W / .00039 .00026 .00018 .08062 .54194 3137 0.57 82.31 1.48 151.961 239.093 7A005@34 34.00 W / .00017 .00021 .00026 .12603 .85026 31.48 0.29 93.94 232 300.408 491.389 7A005@26 26.00W / .00085 .00032 .00053 .23127 1.56251 31.52 0.25 85.84 4.25 353.191 328.642 7A005@31 31.00W / .00021 .00034 .00039 .19997 1.35388 31.59 0.23 95.15 3.68 287.008 277.772 7Z006@46 46.00 W / .00014 .00021 .00044 .23435 1.58765 31.61 0.18 97.01 4.31 540.190 1314.648 7Z006@48 48.00 W / .00044 .00037 .00036 .21017 1.42715 31.68 0.26 91.30 3.86 278.114 341.948 7Z006@49 49.00 W / .00007 .00032 .00027 .11452 .77910 31.74 0.25 97.17 210 174.923 299.957 7Z006@42 42.00W / .00050 .00059 .00092 .46456 3.16649 31.80 0.16 95.16 8.54 384.236 336.326 7Z006@47 47.00 W / .00005 .00037 .00041 .19475 1.32977 31.86 0.20 98.55 3.58 257.552 275.329 7A005@33 33.00 W / .00017 .00018 .00038 .15633 1.06830 31.88 0.25 95.15 2.87 421.372 702.475 7Z006@43 43.00 W / .00095 .00153 .00077 .38525 263579 31.92 0.16 90.01 7.08 123.652 36.398 7A005@30 30.00 W / .00063 .00024 .00040 .18459 1.26407 31.95 0.27 86.88 3.39 381.638 586.943 7A005@29 29.00 W / .00027 .00000 .00020 .14020 .96093 31.98 0.28 92.09 258 360.897 0.000 7A005@39 39.00 W / .00006 .00025 .00018 .07951 .54587 32.03 0.49 96.24 1.46 153.143 238.735 7A005@37 37.00 W / .00023 .00021 .00020 .07011 .48230 32.09 0.50 87.15 1.29 166.598 243.637 7Z006@41 41.00 W / .00148 .00085 .00159 .84235 5.79823 3211 0.15 92.62 15.48 483.787 253.985 7Z006@50 50.00 W / .00040 .00013 .00073 .38729 2.66736 3213 0.16 95.36 7.12 1503.109 4592.216 7Z006@44 44.00W / .00035 .00026 .00036 .18101 1.24852 32.18 0.24 91.99 3.33 345.107 510.859 7A005@36 36.00 W / .00044 .00016 .00028 .13011 .89827 3221 0.37 87.14 2.39 392.909 871.446 7A005@40 40.00W / .00017 .00020 .00017 .08759 .60935 32.45 0.42 92.16 1.61 213.816 378.915 7A005@35 35.00 W / .00020 .00050 .00036 .18092 1.26377 32.58 0.27 95.20 3.33 177.574 100.003 7A005@28 28.00 W / .00056 .00033 .00035 .17249 1.20525 32.59 0.26 87.54 3.17 258.148 211.511 . .00948 .00892 .01087 5.44058 37.16882 Information on Analysis Sample Material G-1 B-34 Muscovite Location Erikas Single x mica

Analyst Project Irradiation J-value Standard msp 2007 c1160-0 28.34

Results 40(r)/39(k) 20 Age 2 39Ar(k) K/Ca 2T

(Ma) S (%.n)

Error Plateau 6.8260 31.85 9.83 100.00 22.598 0596

0.46% 0.50% 25

External Error 0.16 206 Statistical T ratio Analytical Error 0.15 3.1354 Error Magnification

Total Fusion Age 6.8318 0.0105 31.87 0.08 25 298.891 64.330

0.15% 0.25%

External Error 0.08 Analytical Error 0.05

G-3 muscovite

Incremental 36Ar(a) 37Ar(ca) 38Ar(cI) 39Ar(k) 4OAr(r) Age 40Ar(r) 39Ar(k) K/Ca 2

7A005@49 49.00 W .00014 .00048 .00036 .15858 1.02163 30.09 0.21 95.63 288 162.750 93.203 7A005@54 54.00W / .00096 .00038 .00031 .14005 .91106 30.39 0.35 76.00 2.54 181.750 142.989 7A005@51 51.00 W / .00015 .00035 .00025 .10548 .69021 30.56 0.28 93.70 1.91 148.038 130.211 7A005@48 48.00 W / .00021 .00048 .00037 .16641 1.08953 30.58 0.24 94.30 3.02 170.715 98.526 7Z006@53 53.00 W / .00067 .00185 .00101 .43392 284188 30.59 0.17 93.13 7.88 115.003 24.799 7A005@42 42.00 W / .00044 .00028 .00047 27045 1.77299 30.62 0.20 92.80 4.91 480.608 813.921 7A005@52 5200 W / .00021 .00055 .00029 .13164 .86654 30.74 0.26 92.87 2.39 116.279 53.182 7A005@46 46.00W / .00058 .00065 .00029 .13326 .87812 30.78 0.35 83.45 2.42 99.917 55.307 7A005@55 55.00W / .00023 .00035 .00027 .12739 .83995 30.79 0.26 92.16 2.31 179.531 187.797 7A005@45 45.00W / .00041 .00043 .00064 .27295 1.80472 30.88 0.19 93.36 4.95 309.029 207.412 7A005@41 41.00 W / .00216 .00093 .00068 .32229 213506 30.94 0.26 76.71 5.85 169.083 62.245 7A005@47 47.00 W / .00039 .00074 .00039 .16535 1.09663 30.97 0.27 90.10 3.00 109.789 46.794 7Z06@59 59.00 W / .00044 .00106 .00050 .20996 1.39665 31.06 0.21 91.17 3.81 97.358 52.307 73306@51 51.00 W / .00058 .00069 .00067 .33267 2.21438 31.09 0.16 92.42 6.04 235.266 179.129 73306@57 57.00W / .00050 .00048 .00062 .28545 1.90524 31.17 0.20 92.37 5.18 292.932 305.728 73306@55 55.00 W / .00046 .00023 .00058 24479 1.63659 31.22 0.23 91.96 4.44 523.683 952179 7Z06@56 56.00 W / .00020 .00330 .00056 .22237 1.48730 31.23 0.24 95.72 4.04 33.063 3.942 73306@54 54.00 W / .00042 .00038 .00058 .20326 1.35963 31.24 0.24 91.31 3.69 259.873 236.158 7A005@43 43.00 W / .00071 .00023 .00046 .22532 1.50721 31.24 0.24 87.44 4.09 490.463 889280 7A005@44 44.00 W / .00031 .00068 .00047 .19835 1.32803 3127 0.20 9321 3.60 142.320 54.604 73306@0 60.00 W / .00014 .00032 .00048 .20364 1.36642 31.33 0.18 96.73 3.70 315.942 479.888 73306@52 5200 W / .00115 .00027 .00075 .33834 2.27765 31.43 0.21 86.67 6.14 605.118 946.777 7A005@50 50.00 W / .00041 .00026 .00030 .14871 1.00272 31.48 0.26 88.94 270 254.618 297.552 73306@58 58.00W / .00026 .00194 .00089 .32052 217558 31.69 0.17 96.16 5.82 80.820 20.578 7A005@53 53.00W .00022 .00057 .00031 .14828 1.02467 32.26 0.28 93.70 2.69 127.280 69.129 Z .01234 .01787 .01247 5.50944 36.63039 Information on Analysis Sample G-3B-35 Material Muscovite

Location Vietnam Single xl Mica

Analyst msp Project Irradiation J-value Standard Maicolm's Mica's c1160-0 28.34 Age 20 39Ar(k) I~ 2 Results 40(r)/39(k) 2 A K/Ca 2 (Ma) S2 (%NO Errar Plateau 6.6514 0.0309 31.06 0.16 9.70 39.931 10.291 0.46% 0.50% 23

External Error 0.16 2.07 Statisticai T ratio Analytical Error 0.14 3.1138 Error Magnification

Total Fusion Age 6.6487 0.0099 31.05 0.08 25 151.072 16.259

0.15% 0.25%

External Error 0.08 Analytical Error 0.05

SPB-1 muscovite

Single Crystal 36Ar(a) 37Ar(ca) 38Ar(cd) 39Ar(k) 4OAr(r) Age 20 4OAr(r) 39Ar(k) K/Ca 2a

Laser Fusion (Ma) (%) (%)

7A005@69 69.00 W / .00061 .00025 .00022 .09797 .60872 29.05 0.33 76.88 2.49 191.723 t 282.555 7Z006@62 62.00 W / .00210 .00019 .00053 .27729 1.72410 29.08 0.26 73.27 7.06 698.015 1082.952 7A005@66 66.00 W / .00173 .00027 .00019 .08811 .54850 29.11 0.58 51.64 2.24 158.855 174.040 7A005@56 56.00 W / .00088 .00040 .00031 .12517 .78200 29.21 0.48 74.84 3.19 152.696 143.669 7Z006@70 70.00 W / .00057 .00054 .00040 .20826 1.30145 29.22 0.25 88.25 5.30 190.170 137.616 7A005@68 68.00 W / .00184 .00027 .00029 .14437 .90301 29.25 0.46 62.20 3.67 257.293 304.575 7A005@70 70.00 W / .00011 .00025 .00014 .06931 .43352 29.25 0.47 92.47 1.76 135.438 165.149 7Z006@63 63.00 W / .00355 .00028 .00036 .15914 .99962 29.37 0.61 48.71 4.05 282.856 492.532 7A005@65 65.00W / .00209 .00031 .00045 .23875 1.49990 29.37 0.35 70.61 6.08 381.908 510.710 7A005@57 57.00 W / .00056 .00034 .00028 .11002 .69210 29.41 0.29 80.29 2.80 158.702 164.621 7A005@67 67.00 W / .00111 .00031 .00023 .09982 .62806 29.42 0.63 65.48 2.54 157.144 144.390 7A005@62 62.00 W / .00110 .00038 .00029 .13472 .84917 29.47 0.40 72.14 3.43 174.274 147.260 7Z006@65 65.00 W / .00188 .00015 .00044 .22176 1.39874 29.49 0.33 71.37 5.64 737.926 1885.639 7A005@60 60.00 W / .00025 .00030 .00043 .18317 1.15565 29.50 0.22 93.60 4.66 300.120 332.726 7A005@59 59.00W / .00084 .00025 .00025 .09956 .62821 29.50 0.52 71.50 2.53 196.178 204.297 7Z006@67 67.00 W / .00084 .00030 .00046 .21096 1.33180 29.52 0.29 83.99 5.37 338.973 511.357 7Z006@66 66.00 W / .00147 .00008 .00024 .10732 .67916 29.59 0.73 60.80 2.73 673.709 3180.167 7Z006@69 69.00 W / .00167 .00029 .00048 .22996 1.45673 29.62 0.28 74.50 5.85 391.807 508.027 7A005@61 61.00W / .00059 .00042 .00040 .17358 1.10051 29.64 0.34 86.02 4.42 201.715 129.877 7Z006@64 64.00 W / .00121 .00015 .00042 .20850 1.32333 29.67 0.26 78.45 5.31 693.800 1515.246 7A005@64 64.00 W / .00078 .00032 .00038 .19531 1.24042 29.69 0.23 84.06 4.97 300.692 268.229 7Z006@61 61.00 W / .00081 .00005 .00020 .09577 .60851 29.71 0.51 71.62 2.44 959.533 6850.749 7Z006@68 68.00 W / .00138 .00016 .00045 .21720 1.38489 29.81 0.29 76.95 553 682.899 2005.409 7A005@63 63.00 W / .00013 .00000 .00021 .10119 .64621 29.86 0.28 94.02 2.57 163.964 0.000 7A005@58 58.00 W / .00132 .00026 .00029 .13283 .85481 30.08 0.35 68.39 3.38 247.588 296.043 2 .02940 .00651 .00835 3.93004 24.77911 Information on Analvsis Sample SPB-1 B-36 Material Muscovite

Location Vietnam Single xA Mica

Analyst msp Project Irradiation i-value Standard Malcolms Micas c!160-0 28.34

Results 40(r)/39(k) 2 Age 20 g 39Ar(k) K/Ca 2a

(Ma) L2 (%.n)

Error Plateau 6.3100 0.0223 2950 40.12 2 100.00 6.846 0.408

0.35% 0.40% 25

External Error 0.12 206 Statistical T ratio

Ar alytical Error 0.10 1.5567 Error Magnification

TotalFusion Age 6.3050 0.0161 29.48 0.10 25 295.675 80.669

0.26% 0.32%

External Error 0.10 Analytical Error 0.07