Stable isotope and fluid inclusion studies of gem-bearing granitic pegmatite-aplite dikes, San Diego Co., California

Abstract

Late Cretaceous, granitic pegmatite-aplite dikes in southern California have been known for gem-quality minerals and as a commercial source of lithium. Minerals, whole-rock samples, and inclusion fluids from nine of these dikes and from associated wall rocks have been analyzed for their oxygen, hydrogen, and carbon isotope compositions to ascertain the origins and thermal histories of the dikes. Oxygen isotope geothermometry used in combination with thermometric data from primary fluid inclusions enabled the determination of the pressure regime during crystallization. Two groups of dikes are evident from their oxygen isotope compositions (δ18Oqtz≃+10.5 in Group A, and ≃+8.5 in Group B). Prior to the end of crystallization, Group A pegmatites had already extensively exchanged oxygen with their wall rocks, while Group B dikes may represent a closer approximation to the original isotopic composition of the pegmatite melts. Oxygen isotope fractionations between minerals are similar in all dikes and indicate that the pegmatites were emplaced at temperatures of about 730 ° to 700 ° C. Supersolidus crystallization began with the basal aplite zone and ended with formation of “quench aplite” in the pocket zone, nearly to 565 ° C. Subsolidus formation of gem-bearing pockets took place over a relatively narrow temperature range of about 40 ° C (approximately 565–525 ° C). Nearly closed-system crystallization is indicated. Hornblende in gabbroic and noritic wall rocks (δDw.r. = −90 to −130) in the Mesa Grande district crystallized in the presence of, or exchanged hydrogen with, meteoric water (δD≃ −90) prior to the emplacement of the pegmatite dikes. Magmatic water was subsequently added to the wall rocks adjacent to the pegmatites. Groups A and B pegmatites cannot be distinguished on the basis of their hydrogen isotope compositions. A decrease in δD of muscovite inward from the walls of the dikes reflects a decrease in temperature. δD values of H2O from fluid inclusions are: −50 to −73 (aplite and pegmatite zones); −62 to −75 (pocket quartz: Tourmaline Queen and Stewart dikes); and −50 ± 4 (pocket quartz from many dikes). The average δ13C of juvenile CO2 in fluid inclusions in Group B pegmatites is −7.9. In Group A pegmatities, δ13C of CO2 is more negative (−10 to −15.6), due to exchange of C with wall rocks and/or loss of 13C-enriched CO2 to an exsolving vapor phase. Pressures during crystallization of the pockets were on the order of 2,100 bars, and may have increased slightly during pocket growth. A depth of formation of at least 6.8 km (sp. gr. of over burden = 3.0, and P fiuid=P load) is indicated, and a rate of uplift of 0.07 cm/yr. follows from available geochronologic data.