Journal of Petrology, Volume 40, Issue 6: June 1999.

Generation of Tonalite and Trondhjemite by Subvolcanic Fractionation and Partial Melting in the Zarza Intrusive Complex, Western Peninsular Ranges Batholith, Northwestern Mexico

MARCUS C. TATE¹, MARC D. NORMAN¹, SCOTT E. JOHNSON^1,2, C. MARK FANNING³ AND J. LAWFORD ANDERSON⁴

¹DEPARTMENT OF EARTH AND PLANETARY SCIENCES, MACQUARIE UNIVERSITY, NORTH RYDE, N.S.W. 2109, AUSTRALIA
²DEPARTAMENTO DE GEOLOGIA, CICESE, KM 107 CARR. ENSENADA-TIJUANA, MEXICO
³RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
⁴DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF SOUTHERN CALIFORNIA, LOS ANGELES, CA 90089-0740, USA

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/⁸⁶Sr_{t = 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/⁸⁶Sr_i 0·703) probably formed in situ by andesite fractionation, whereas spatially associated trondhjemite (A/CNK > 0·98, ⁸⁷Sr/⁸⁶Sr_i 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.

Keywords: andesite;cone-sheets;cumulates; partial melting; tonalite-trondhjemite

Pages 983-1010