Generation of tonalite and trondhjemite by subvolcanic fractionation and partial melting in the Zarza Intrusive Complex, western Peninsular Ranges Batholith, northwestern Mexico

The Early Cretaceous (similar to 115 Ma) Zarza Intrusive Complex is a small (<10 km(2)), bimodal ring complex that may represent a magmatic microcosm of the western Peninsular Ranges batholith. Its tholeiitic gabbro bosses (2 5% 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 greater than or equal to 55%, MgO < 3.1%, Ni similar to 30 ppm). Quenched cone-sheets are polymorphic (olivine -pyroxene- or hornblende-bearing) and share similar chemical and isotopic c ompositions (epsilon(Nd) +7, Sr-87/Sr-86(t = 115) < 0.704) that preclude ex tensive sediment contamination. Their calc-alkaline basalt parents apparent ly contained very different volatile concentrations (similar to 3-7 wt % H2 O) inherited from various equilibria between subduction-related aqueous flu ids, and depleted lherzolite in the upper mantle. Recharge and/or dominant ferromagnesian mineral fractionation at similar to 0.8 GPa (>28 km) depth b est explains subsequent differentiation towards high-Al andesite. Contempor aneous tonalite (SiO2 64-74%, molar Al2O3/(CaO + Na2O + K2O) [A/CNK] > 1.0, Sr-87/Sr-86(i) 0.703) probably formed in situ by andesite fractionation, w hereas spatially associated trondhjemite (A/CNK > 0.98, Sr-87/Sr-86(i) 0.70 2) is more consistent with 8-19% dehydration melting of metabasite in the c ontact aureole. Enrichments of incompatible K2O, Ba, Rb and Th in all silic a-saturated rocks from the western part of the batholith can be explained b y mixing between different proportions of fractionated and partially melted end-members generated within thick oceanic arc basement.