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Description
The Harper Creek gneiss (HCg) crops out as four distinct sheet-like bodies along the western margin of the Cuyamaca Laguna Mountains shear zone (CLMSZ) which parallels the northwest-striking axis of the Cretaceous Peninsular Ranges Batholith (PRB). The HCg is largely Early Jurassic in age (193 ± 16 Ma) and clearly predates the emplacement of the Cretaceous aged Peninsular Ranges batholith (PRB) ( 140 — 89 Ma). The four HCg bodies studied are the Viejas (VC), Descanso (DC), Scove Canyon (SCC), and Mount Laguna (MLC) complexes. The purpose of this study is: (1) to describe the mineralogy of the HCg rocks, (2) to constrain the source origin by using trace elements and Pb-isotope data as tracers, and (3) to broadly constrain the depth of emplacement of the HCg plutons. The HCg varies from rock types with relict igneous hypidiomorphic to allotriomorphic granular textures to rocks with foliated gneissic and mylonitic fabrics. The VC is predominantly a biotite tonalite with various amounts of amphibole or muscovite. The DC and MLC are on, average, ± muscovite-biotite granitoids except MLC15 which is a muscovite-cordierite-biotite granite. The SCC is a muscovite biotite quartz-rich granitoid. The HCg complexes have been variably metamorphosed in the lower-upper greenschist, epidote-amphibolite, and amphibolite facies. The HCg as a whole is predominantly calc-alkaline, and generally define near linear trends on Harker variation diagrams. On Debon -LeFort A vs. B discrimination plot the DC, SCC and MLC form magmatic series trends completely within the peraluminous field, whereas the VC and GMC form magmatic series trends vary from the metaluminous to peraluminous fields. The HCg as a whole falls in fields defined by volcanic arc (VAG) and syncollisional granites (syn-COLYG) on Pearce trace element discrimination plots. The REE signatures of the HCg are moderately to highly enriched in LREE (10to 110 X chondrites), flat to slightly depleted in HREE (7 to 20 X chondrites), and display a weak to average negative Eu anomaly. These patterns suggest that the HCg source region contains residual hornblende and plagioclase (amphibolitic assemblages). Trace element spider diagrams from the HCg are similar to upper crustal rocks (Julian Schist-like rocks). They are typically enriched in LILs and LREE, depleted in Sr-Eu-Ti, and flat and depleted in HREE. The Pb isotope data for all four localities define a near vertical linear array in Pb-Pb isotope space suggestive of mixing between a depleted MORB-like source and a more radiogenic crustal source component. The occurrence of magmatic muscovite and biotite suggests that the minimum depth of emplacement of the HCg melts occurred at about P = 4.0 kb (14 km) or greater. The mineral assemblages in a granitic xenolith from MLC suggest that some HCg plutons could have been emplaced at pressures as deep as 5.5 kb (19 km). Retrograde metamorphic assemblages indicate that the HCg rocks adjusted from upper amphibolite to lower-upper greenschist metamorphic conditions during uplift. Within the context of the current tectonic models for the region of Peninsular California and western Mexico, the HCg is defined as a composite of compositionally and temporally similar but distinct prebatholithic magmatic bodies that formed on the leading edge of the convergent continental margin of western North America. During Early Jurassic time mantle-derived magmas intruded the Triassic sedimentary apron (which includes the Julian Schist) and partially melted these crustal rocks. The mixing of the crustal component with the mantle-derived component resulted in HCg-type plutons which were subsequently deformed.
