Oxygen and hydrogen isotope evidence for large-scale circulation and interaction between ground waters and igneous intrusions, with particular reference to the San Juan Volcanic Field, Colorado

Abstract

The 33 to 25 m.y.-old intrusive and volcanic rocks from western San Juan Mountains (Silverton-Ouray area of Colorado) are all abnormally low in O¹⁸ relative to most igneous rocks, particularly along the eastern edge of the Silverton Caldera where the average δO¹⁸ = -5. The only exceptions to this are certain plutons emplaced jnto Paleozoic and Mesozoic sedimentary rocks. Therefore, although in the volcanic rocks very large-scale convective circulation systems involving heated meteoric ground waters were established by the epizonal igneous intrusions, this did not occur in the less permeable sedimentary sections. The very low δD values of the hydrous minerals in the igneous rocks confirm these conclusions (δD = -137 to -150). The same processes also occurred in the vicinity of the 29 m.y.-old Alamosa stock in the eastern San Juan Mountains, but the δO¹⁸ depletion effects are not so extreme and the δD values are much higher (-101 to -118). This seems to imply that in the mid-Tertiary, the meteoric waters were about 35 per mil different in these two areas, even though they are only 70 miles apart. The meteoric-hydrothermal alteration processes that have affected these types of rocks can in certain circumstances produce almost complete oxygen exchange in plagioclase phenocrysts while preserving igneous textural features and delicate oscillatory zoning in the plagioclase. With two exceptions all of the low-O¹⁸ igneous rocks throughout the world are of late Mesozoic to Tertiary age; these are now known to be very extensive and quite common in all volcanic fields where there are shallow igneous intrusions. In Precambrian rocks, however, the reverse is true. Alkali feldspars from the red-rock granophyres and granites in the Muskox, Bushveld, and Duluth Complexes, and in the St. Francois and Keweenawan volcanic terranes are commonly higher in O¹⁸ than coexisting quartz. Although this could have resulted from much higher-O¹⁸ meteoric waters in the Precambrian (which would imply that the oceans also were much higher in O¹⁸), it seems more likely that this is due to very low-temperature (~150°C) exchange with ground-water brines that circulated through these rocks for long periods of time. All these alkali feldspars are turbid and contain disseminated hematite dust; this high oxidation state is readily explained if the alteration occurs at low temperatures. It would also explain why Rb-Sr ages on these rocks are generally younger and much more variable than are the Pb-U zircon ages, It is because zircon is very resistant to exchange, and the turbid feldspars and altered mafic minerals are not.