Petrogenetic modeling and stable isotopic evaluation of anorthositic and jotunitic to syenitic magma series in the San Gabriel anorthosite complex, southern California

Abstract Two magmatic series have been recognized in the 1.12 Ga San Gabriel anorthosite-syenite complex in the western San Gabriel Mountains of southern California: one anorthositic and the other jotunitic to syenitic. The anorthosite series consists of anorthosite (avg. ∼ An48), gabbroic anorthosite, and gabbro. Anorthositic rocks are bordered on their southern and western margins by a mafic jotunite-syenite series that forms slightly curved trends on Harker variation diagrams. Volumetrically minor ultramafite occurs as dikes and sheets that are highly enriched in Fe, Ti, and P and have geochemical signatures suggesting affinity with the jotunite-syenite series. Although high-grade regional 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 anorthosite rocks and near-complete replacement of primary pyroxene by actinolitic aggregates in all rock units. Oxygen isotopic fractionations between coexisting secondary calcite and host feldspar indicate disequilibrium. It is concluded that δ18O values of primary minerals are not far-shifted from magmatic values, and that the effects of hydrothermal alteration were generally localized. Moreover, the alteration has not affected the elemental composition of the two series allowing their use in evaluation of magmatic evolution. The anorthosite series and the jotunite-syenite series are modeled both as (1) comagmatic and descending from a broadly dioritic parent, and (2) as non-comagmatic and descending from a gabbroic anorthosite parent for the anorthosite series and from a jotunitic parent for the jotunite-syenite series. In both cases, rocks of the anorthosite series are considered to be entirely cumulate in origin. In the jotunite-syenite series, ultramafite is regarded as an early-formed, sorted cumulate similar in composition to mafic jotunite but lacking feldspar, and strongly layered mafic jotunite is also regarded as a cumulate. Felsic jotunite and syenite may more closely approximate magmas, but the small variation in Mg number suggests that crystal sorting may be responsible for much of the compositional variation in the series. High K, Ba, Rb and Eu/Eu∗ in some syenite may indicate accumulation of potassium feldspar in these rocks. During anorthosite crystallization, magmas are envisioned to follow an SiO2-depletion, Feenrichment trend that is reversed by the crystallization of mafic jotunite in the comagmatic case, but may eventually result 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, possibly due to the strong effect of apatite and uncertainty in apatite partition coefficients. Although the unconstrained choice of parent magma(s) composition is a deficiency in both models, the failure of the comagmatic model to describe REE variations is not related to choice of parent magma composition. The non-comagmatic model to describe REE variations is not related to choice of parent magma composition. The non-comagmatic model can best explain all compositional variations, largely as a result of the additional freedom of choice of parent magma composition. In addition, the limited spatial occurrence of the jotunite-syenite unit and the lack of jotunite dikes in the complex may support the intrusion of jotunite-syenite along the anorthosite-country rock contact as a pluton after, or possibly during, the final stages of anorthosite crystallization. On the basis of geochemical modeling and field relations, we favor the crystallization of the two magma series as non-comagmatic with anorthositic rocks forming as cumulates from a gabbroic anorthosite parent magma and ultramafite, jotunite, and syenite forming from a jotunitic parent magma.