Origin of Mafic Enclaves in the Dinkey Creek Pluton, Central Sierra Nevada Batholith, California |
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| Authors: | DORAIS MICHAEL J; WHITNEY JAMES A; RODEN MICHAEL F |
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| Institution: | Department of Geology, University of Georgia Athens, Georgia 30601 |
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| Abstract: | Two types of mafic enclaves occur in the Dinkey Creek pluton:ubiquitous microgranular enclaves, and rare gabbroic enclaves.Common petrographic features of the microgranular enclaves are:(1) fine grain-size, (2) abundant acicular apatite, and (3)plagioclase zoned from bytownitic cores to andesine-labradoriterims, with sharp boundaries between these main zones. Subordinateoscillatory variations are commonly superimposed on both coresand rims. It has been found by secondary ion mass spectrometrythat the rims are identical in major and trace element compositionto plagioclase in the tonalite, which suggests crystallizationfrom the same or similar magmas. The gabbroic enclaves are composedpredominantly of hornblende (50–85%) and appear to bemagmatic segregations. The microgranular enclaves and host rocks display two convergingtrends on silica variation diagrams for Fe2O3, TiO2, Al2O3,Zn, and Zr. The dominant trend is defined by small microgranularenclaves, by samples from a large (20 m?30 m) microgranularenclave, and by the Dinkey Creek tonalites and granodiorites.The subordinate trend covers tholeiltic dikes and tonalitich and converges with the Dinkey Creek host rocks at 61 wt.%SiO2 Alkali and alkaline earth elements exhibit greater variabilitythan the above constituents and appear to be either enrichedor depleted as required for equilibrium with the host rocks.Low CaO and Sr concentrations in small enclaves (<30 cm)apparently reflect a lower modal abundance of calcic plagioclaseand more sericitization of this feldspar as compared with theplagioclase of the large microgranular enclave. The large enclaveis also richer in MgO than the small enclaves. With the exceptionof the alkali elements, the major element compositions of themicrogranular enclaves approach high-Al basaltic to andesiticcom positions. In one analyzed microgranular enclave, low La/Cerelative to chondrites and more abundant HREE than in othermicrogranular samples suggest that it may also contain minorcumulus hornblende. The petrographic and whole-rock geochemical relations, and theplagioclase compositions in the microgranular enclaves and theirhost rocks, indicate that the microgranular enclaves representmixtures of quenched basalts and Dinkey Creek tonalites. Itappears that dikes of high-alumina basalt were intruded intothe lower, tonalitic portions of the Dinkey Creek pluton, wherethey were partially quenched along an interface with overlyingtonalitic magma. Large portions of residual liquid in the partiallyquenched basalts permitted mixing with the overlying magma toform a hybrid zone. This zone was then disaggregated, yieldingthe enclaves, and they were dispersed throughout the upper partof the Dinkey Creek magma chamber. Subsequent crystallizationof tonalitic melt within the enclaves produced the zoned plagioclaseand re-equilibrated hornblende and biotite in the enclaves tothe Dinkey Creek magmatic conditions. Scouring disrupted hornblende-richmagmatic segregations and produced the gabbroic enclaves. |
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