PETROGENESIS OF THE HIGHLY POTASSIC 1.42 GA BARREL SPRING PLUTON, SOUTHEASTERN CALIFORNIA, WITH IMPLICATIONS FOR MIDPROTEROZOIC MAGMA GENESIS IN THE SOUTHWESTERN USA

Syenites from the Barrel Spring pluton were emplaced in the Early Prot erozoic Mojave crustal province of southeastern California at 1.42 Ga. All rocks, even the most mafic, are highly enriched in incompatible e lements (e.g. K2O 4-12 wt%, Rb 170-370 ppm, Th 12-120 ppm, La 350-1500 x chondrite, La/Yb-n 35-100). Elemental compositions require an incom patible element-rich but mafic (or ultramafic) source. Trace element m odels establish two plausible sources for Barrel Spring magmas: (1) LR EE enriched garnet websterite with accessory apatite +/- rutile (enric hed lithospheric mantle), and (2) garnet amphibolite or garnet-hornble nde granulite with enriched alkali basalt composition, also with acces sory apatite +/- rutile (mafic lower crust). Nd and Pb isotopic ratios do not distinguish a crust vs mantle source, but eliminate local Moja ve province crust as the principal one, and indicate that generation o f the enriched source occurred several hundred million years before em placement of the Barrel Spring pluton. 1.40-1.44 Ga potassic granites are common in southeastern California, suggesting a genetic link betwe en the Barrel Spring pluton and the granites; however, although the sa me thermal regime was probably responsible for producing both the gran itic and syenitic magmas, elemental and isotopic compositions preclude a close relationship. Isotopic similarity of the Barrel Spring pluton to 1.40-1.44 Ga granites emplaced in the Central Arizona crustal prov ince to the east may imply that a common component was present in the lithosphere of these generally distinct regions.