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Description
Dark, fine-grained inclusions are not uncommon to the various intrusive bodies of the Peninsular Ranges batholith. They are most abundant in plutons of quartz diorite-tonalite composition. One such unit of this nature is the Las Blancas pluton which is located approximately 48 km east-northeast of San Diego, California. The portion of the Las Blancas pluton studied is comprised of four plutonic rock types: (1) leucogabbronorite, (2) quartz diorite, (3) abundant tonalite, and (4) granodiorite. Mineralogic and petrographic similarities, spatial relationships, and smooth geochemical variations indicate that the plutonic units are comagmatic and were produced as a result of a differentiation process. Dark inclusions are found within all four of the plutonic rocks. They are diorite to quartz diorite in composition and exhibit relict igneous textures. Modal and geochemical analyses indicate differentiation within the inclusion suite. Comparison of mineral phases, textural features, and geochemical patterns between the inclusions and the plutonic series indicates a comagmatic relationship. The comagmatic relationship and a relict porphyritic texture indicate that the inclusions are a chilled phase of a differentiated melt. A fractionated parental gabbroic melt was injected into a cooler region resulting in a partial quenching and a chilled margin of diorite composition. Within this new region the remaining portions of the daughter melt continued to fractionate, during which time successive differentiated pulses were injected upward tearing off and including portions of the chilled margin. Differentiation was concluded during a period of tectonic quiescence with the crystallization of granodiorite. Formation of rims occurred as inclusions began to act as nucleating sites generally for initial precipitation of plagioclase. Disequilibrium between the melt and outer margins of the newly-formed plagioclase phenocrysts resulted in plagioclase back-reacting with the melt forming embayed regions which were filled by the interstitial phases of amphibole and/or biotite. Following the crystallization of this plutonic series a second major pulse of gabbroic melt derived from the parental source was emplaced into the differentiated sequence. Fractionation of the gabbroic melt at this level resulted in the leucogabbronorite.
