Journal of Petrology | Volume 40 | Number 6 | Pages 983-1010 | 1999
© Oxford University Press 1999
Generation of Tonalite and Trondhjemite by Subvolcanic Fractionation and Partial Melting in the Zarza Intrusive Complex, Western Peninsular Ranges Batholith, Northwestern Mexico
1 Department of Earth and Planetary Sciences, Macquarie University North Ryde, N.S.W. 2109, Australia
2 Departamento De Geologia, Cicese, KM 107 CARR. Ensenada-Tijuana Mexico
3 Research School of Earth Sciences, The Australian National University Canberra, A.C.T. 0200, Australia
4 Department of Earth Sciences, University of Southern California Los Angeles, Ca 90089–0740, USA
Received April 20, 1998; Revised typescript accepted January 8, 1999
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Abstract |
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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 (
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.
KEY WORDS: andesite; cone-sheets; cumulates; partial melting; tonalite–trondhjemite
* Corresponding author. Present address: Department of Geology, The University of Newcastle, Callaghan, N.S.W. 2308, Australia. Fax: 61 29850 8428. e-mail mtate{at}laurel.ocs.mq.edu.au
Present address: Centre for Ore Deposit Studies (CODES), School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tas. 7000, Australia.