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- Title
Generation of Tonalite and Trondhjemite by Subvolcanic Fractionation and Partial Melting in the Zarza Intrusive Complex, Western Peninsular Ranges Batholith, Northwestern Mexico.
- Authors
TATE, MARCUS C.; NORMAN, MARC D.; JOHNSON, SCOTT E.; FANNING, C. MARK; ANDERSON, J. LAWFORD
- Abstract
The Early Cretaceous (~115 Ma) Zarza Intrusive Complex is a small (<10 km2), bimodal ring complex that may represent a magmatic microcosm of the western Peninsular Ranges batholith. Its tholeiitic gabbro bosses (25% by area; Al2O3 > 17 wt %, Sr < 463 ppm) formed at subvolcanic depths <0·2 GPa (8 km) by >30% plagioclase accumulation from andesitic magma batches now preserved as cone-sheets (63%; SiO2 >= 55%, MgO < 3·1%, Ni ~30 ppm). Quenched cone-sheets are polymorphic (olivine-pyroxene- or hornblende-bearing) and share similar chemical and isotopic compositions ([epsilon]Nd +7, 87Sr/86Srt = 115 < 0·704) that preclude extensive sediment contamination. Their calc-alkaline basalt parents apparently contained very different volatile concentrations (~3-7 wt % H2O) inherited from various equilibria between subduction-related aqueous fluids, and depleted lherzolite in the upper mantle. Recharge and/or dominant ferromagnesian mineral fractionation at ~0·8 GPa (>28 km) depth best explains subsequent differentiation towards high-Al andesite. Contemporaneous tonalite (SiO2 64-74%, molar Al2O3/(CaO + Na2O + K2O) [A/CNK] > 1·0, 87Sr/86Sri 0·703) probably formed in situ by andesite fractionation, whereas spatially associated trondhjemite (A/CNK > 0·98, 87Sr/86Sri 0·702) is more consistent with 8-19% dehydration melting of metabasite in the contact aureole. Enrichments of incompatible K2O, Ba, Rb and Th in all silica-saturated rocks from the western part of the batholith can be explained by mixing between different proportions of fractionated and partially melted end-members generated within thick oceanic arc basement.
- Subjects
MEXICO; TONALITE; TRONDHJEMITE; BATHOLITHS; ANDESITE
- Publication
Journal of Petrology, 1999, Vol 40, Issue 6, p983
- ISSN
0022-3530
- Publication type
Article
- DOI
10.1093/petroj/40.6.983