PETROGENETIC MODELING AND STABLE ISOTOPIC EVALUATION OF ANORTHOSITIC AND JOTUNITIC TO SYENITIC MAGMA SERIES IN THE SAN-GABRIEL ANORTHOSITE COMPLEX, SOUTHERN CALIFORNIA

Two magmatic series have been recognized in the 1.12 Ga San Gabriel an orthosite-syenite complex in the western San Gabriel Mountains of sout hern California: one anorthositic and the other jotunitic to syenitic. The anorthosite series consists of anorthosite (avg. similar to An(48 )), gabbroic anorthosite, and gabbro. Anorthositic rocks are bordered on their southern and western margins by a mafic jotunite-syenite seri es that forms slightly curved trends on Harker variation diagrams. Vol umetrically minor ultramafite occurs as dikes and sheets that are high ly enriched in Fe, Ti, and P and have geochemical signatures suggestin g affinity with the jotunite-syenite series. Although high-grade regio nal metamorphism has not affected the complex, it has experienced post -emplacement deformation and most rocks exhibit variable hydrothermal alteration. Alteration includes extensive sericitization in the anorth osite rocks and near-complete replacement of primary pyroxene by actin olitic aggregates in all rock units. Oxygen isotopic fractionations be tween coexisting secondary calcite and host feldspar indicate disequil ibrium. It is concluded that delta(18)O values of primary minerals ai- e not far-shifted from magmatic values, and that the effects of hydrot hermal alteration were generally localized. Moreover, the alteration h as not affected the elemental composition of the two Series allowing t heir use in evaluation of magmatic evolution. The anorthosite series a nd the jotunite-syenite series are modeled both as (1) comagmatic and descending from a broadly dioritic parent, and (2) as non-comagmatic a nd descending from a gabbroic anorthosite parent for the anorthosite s eries and from a jotunitic parent for the jotunite-syenite series. In both cases, rocks of the anorthosite series are considered to be entir ely cumulate in origin. In the jotunite-syenite series, ultramafite is regarded as an early-formed, sorted cumulate similar in composition t o mafic jotunite but lacking feldspar, and strongly layered mafic jotu nite is also regarded as a cumulate. Felsic jotunite and syenite may m ore closely approximate magmas, but the small variation in Mg number s uggests that crystal sorting may be responsible for much of the compos itional variation in the series. High K, Ba, Rb and Eu/Eu in some sye nite may indicate accumulation of potassium feldspar in these rocks. D uring anorthosite crystallization, magmas are envisioned to follow an SiO2-depletion, Fe-enrichment trend that is reversed by the crystalliz ation of mafic jotunite in the comagmatic case, but may eventually res ult in the formation of ore bodies in the non-comagmatic model. Major- element models are applied successfully for both hypotheses; however, the REE data cannot be successfully modeled in the comagmatic case, po ssibly due to the strong effect of apatite and uncertainty in apatite partition coefficients. Although the unconstrained choice of parent ma gma(s) composition is a deficiency in both models, the failure of the comagmatic model to describe REE variations is not related to choice o f parent magma composition. The non-comagmatic model to describe REE v ariations is not related to choice of parent magma composition. The no n-comagmatic model can best explain all compositional variations, larg ely as a result of the additional freedom of choice of parent magma co mposition. In addition, the limited spatial occurrence of the jotunite -syenite unit and the lack of jotunite dikes in the complex may suppor t the intrusion of jotunite-syenite along the anorthosite-country rock contact as a pluton after, or possibly during, the final stages of an orthosite crystallization. On the basis of geochemical modeling and fi eld relations, we favor the crystallization of the two magma series as non-comagmatic with anorthositic rocks forming as cumulates from a ga bbroic anorthosite parent magma and ultramafite, jotunite, and syenite forming from a jotunitic parent magma.