7. DISCUSSION
7.3. Interpretation of mineral and whole-rock compositions of Group IIHN xenoliths
A detailed crystallization sequence for this group of xenoliths is difficult to establish due to the high proportions of hornblende. However, at least two crystallization events can be distinguished: (1) crystallization of Cr-spinel, olivine, plagioclase, clinopyroxene, and orthopyroxene, and (2) reaction of an evolved water-rich liquid with the pre-existing minerals to produce poikilitic hornblende and phlogopite. The high anorthite contents of plagioclase (An88) suggest that the original magma was more water-rich than most xenoliths of Group IICL.
Geochemical evidence for melt and fluid migration
The bulk-rock compositions of these xenoliths are characterized by low concentrations of incompatible elements and high abundances of Ni and high Ca/Na, when compared to the TSPC basalts (Figs. 15 and 16). As hornblende crystallization apparently involved reactions that consumed olivine and plagioclase, it is difficult to establish from the observed modal abundaces whether the low incompatible element concentrations of these xenoliths are due solely to plagioclase and olivine accumulation, or if melt migration was also important. Ratios of incompatible elements such as P/Zr and P/Y values are within those of the TSPC basalts, which suggests that if migration of interstitial liquid occurred, it was prior to apatite crystallization. In contrast, their Rb/Y and K/P values are typically higher than those of the TSPC basalts. Using the same arguments presented for the Group IICL xenoliths, we suggest that a fluid fluxed the interstitial liquid of these xenoliths.
Textural and mineralogical evidence for melt and fluid migration
Because plagioclase inside hornblende shows bent twins (Fig. 7) it seems that hornblende (and probably phlogopite) crystallized after some deformation had already occurred. This is consistent with the interpretation that compaction of the cumulate pile and expulsion of interstitial melt is partly responsible for the low incompatible element concentrations of these xenoliths. We propose that after some interstitial melt escaped from the crystal-mush, a fluid enriched the residual liquid in volatile components including K, Rb, and H2O. This triggered reactions between the cumulus minerals and the liquid and produced hornblende and phlogopite with high mg-numbers and Cr2O3 contents. The process proposed here might be analogous to the situation described by Boudreau (1999) for the Olivine-Bearing Zone I of the Stillwater Complex (Montana). He interpreted the occurrence of hornblende and biotite mantling resorbed plagioclase and olivine as due to fluid fluxing and reactions.
Additional evidence for arrival of an aqueous fluid are the halogen contents of apatite (Fig. 14). Apatites from this group of xenoliths have high Cl/F values, and overlap with the samples of Group IICL for which fluid fluxing was proposed. The few available stable isotope analyses of this group of xenoliths (sample Hx14v: hornblende, δ18O = 5.3;
plagioclase, δ18O = 6.1; bulk-rock, δD = -62, all values relative to SMOW; B.S. Singer, unpublished data) suggest that the fluids that fluxed the xenoliths were magmatic and not meteoric (e.g., Taylor & Forester, 1979).
7.4. Implication for the presence of hornblende and phlogopite in calc-alkaline gabbroic rocks
The three groups of gabbroic xenoliths have significants amounts of hornblende and phlogopite, either as small crystals filling microfractures, or as large poikilitic post-cumulus crystals that can make up >50 vol. % of the rock. In this respect they are not unusual, and a survey of the literature shows that the majority of calc-alkaline gabbroic xenoliths and plutons have important amounts of hornblende (xenoliths: Aoki, 1971, Itinome-gata, Japan; Arculus & Wills, 1980, Lesser Antilles; Conrad & Kay, 1984, Adak, Aleutian arc; Grove & Donnelly-Nolan, 1986, Medicine Lake, California; Beard, 1986, compilation from different sites; Yagi & Takeshita, 1987, Japan; Fichaut et al., 1989, Martinica, Lesser Antilles; Beard & Borgia, 1989, Arenal volcano, Costa Rica; Heliker, 1995, Mt. St. Helens; Hickey-Vargas et al., 1995, Calbuco volcano, southern Chile;
gabbroic plutons: Smith et al., 1983, Peninsular Ranges batholith, California; Ulmer et al., 1983, Adamello batholith, Italy; Fabriès et al., 1984, Saint Quay-Portrieux intrusion, Britanny, France; Otten, 1984, Artfjället gabbro, Swedish Caledonides; Regan, 1985, Coastal batholith of Peru; Whalen, 1985, Uasilau-Yau Yau Intrusive Complex, New Britain; Beard, 1986, compilation from different sites; Himmelberg et al., 1987, Yakobi intrusion, Alaska; Beard & Day, 1988, Smartville Complex, California; DeBari &
Coleman, 1989, Tonsina Complex, Alaska; Springer, 1989, Pine Hill Complex, California;
Kepezhinskas et al., 1993, Kamchatka; DeBari, 1994, Fiambalá intrusion, Argentina;
Tepper, 1996, Chilliwack batholith, Washington; Sisson et al., 1996, Hornblende gabbro sill, California). This is in marked contrast with the rare occurrences of hornblende phenocrysts in basalts or basaltic andesites in arc volcanoes world wide (Sigurdsson &
Shepherd, 1974, Kick’em-Jenny Volcano, Lesser Antilles; Arculus, 1976, Grenada, Lesser Antilles; Arculus et al., 1976, Bogoslof Volcano, Alaska; Luhr & Carmichael, 1985, Cerro la Pilita, Mexico; Peterson & Rose, 1985, Ayarza caldera, Guatemala; Rose, 1987, Santa María Volcano, Guatemala). The occurrence and compositions of hornblende (e.g., high mg-number and Cr2O3 contents) in gabbroic xenoliths has led to some authors (e.g., Conrad & Kay, 1984; Yagi & Takeshita, 1987; Beard & Borgia, 1989) to propose that it was an early crystallizing mineral, with the implication that it might be responsible for the calc-alkaline trend of subduction zone magmas (e.g., Yagi & Takeshita, 1987).
Experiments on basaltic and basaltic andesitic water-bearing compositions have shown that, in some instances, hornblende can be a near liquidus mineral even at low pressures (<
3 kbar; e.g., Sisson & Grove, 1993; Barclay & Carmichael, in prep). This has led to the interpretation that the paucity of hornblende phenocrysts in mafic lavas is due to an increase in magma crystallinity once hornblende crystallizes, thus hornblende-bearing mafic magmas are unable to erupt (Barclay & Carmichael, in prep). The petrological, mineralogical and geochemical characteristics of the San Pedro xenoliths suggest that the large amount of hornblende in gabbroic rocks can be due to open-system percolation in crystal piles. Aside from the possibility of hornblende crystallizing due to protracted closed-system crystallization, we propose that migration of evolved melt (and fluid) in plutonic systems can produce reactions with early crystallized minerals (olivine, Cr-spinel, pyroxenes and plagioclase) leading to the production of large proportions of hornblende (and phlogopite) with compositions that resemble those of early crystallization from mafic magmas (e.g., high mg-numbers and Cr2O3 contents).
8. CONCLUSIONS
A detailed petrographic and compositional study of two groups of gabbroic xenoliths from the Tatara-San Pedro Complex has shown that these xenoliths record multistage crystallization histories involving reactions related to melt migration. Reaction of mafic cumulus minerals (Cr-spinel, olivine, pyroxenes and plagioclase) with percolating evolved melts and fluids triggered crystallization of hornblende and phlogopite. We suggest that the higher abundance of these hydrous minerals in calc-alkaline gabbros compared to basalts or basaltic andesites can be explained by melt migration and reaction processes within the cumulate piles of magma chambers.
9. ACKNOWLEDGEMENTS
I would like to thank: A.Wulff and M.Rhodes for the XRF analyses, P. Voldet for the ICP-AES analyses, F. Ramos for the TIMS analyses, Y.Vinzce and T.Thon-That for the
40Ar/39Ar analyses, and F. Parat for the electron microprobe analyses of apatite. I’m also grateful to J. Barclay for making me available her unpublished experimental data and for many discussions about occurrences of horblende-bearing mafic lavas. Field work was funded by the ASSN.
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