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The Early Cretaceous (∼115 Ma) Zarza Intrusive Complex is a small (&lt;10 km 2 ), bimodal ring complex that may represent a magmatic microcosm of the western Peninsular Ranges batholith. Its tholeiitic gabbro bosses (25% by area; Al 2 O 3  &gt; 17 wt %, Sr &lt; 463 ppm) formed at subvolcanic depths &lt;0.2 GPa (8 km) by &gt;30% plagioclase accumulation from andesitic magma batches now preserved as cone-sheets (63%; SiO 2  ≥ 55%, MgO &lt; 3.1%, Ni ∼30 ppm). Quenched cone-sheets are polymorphic (olivine–pyroxene- or hornblende-bearing) and share similar chemical and isotopic compositions (ε Nd  +7,  87 Sr/ 86 Sr t = 115  &lt; 0.704) that preclude extensive sediment contamination. Their calc-alkaline basalt parents apparently contained very different volatile concentrations (∼3–7 wt % H 2 O) 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 (&gt;28 km) depth best explains subsequent differentiation towards high-Al andesite. Contemporaneous tonalite (SiO 2  64–74%, molar Al 2 O 3 /(CaO + Na 2 O + K 2 O) [A/CNK] &gt; 1.0,  87 Sr/ 86 Sr i  0.703) probably formed in situ by andesite fractionation, whereas spatially associated trondhjemite (A/CNK &gt; 0.98,  87 Sr/ 86 Sr i  0.702) is more consistent with 8–19% dehydration melting of metabasite in the contact aureole. Enrichments of incompatible K 2 O, 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.

journal of petrology

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