Geochemical and isotopic studies of the Lady of the Lake Intrusion and associated tobacco root Batholith: Constraints on the genetic relation between Cretaceous mafic and silicic magmatism in Southwestern Montana

Abstract

Small volumes of alkalic mafic intrusions are spatially associated with Cretaceous to Early Tertiary granodioritic to granitic intrusions in the batholithic province of southwestern Montana. The mafic rocks generally occur near the contacts of the Boulder, Pioneer, and Tobacco Root Batholiths with country rocks, but their genetic relation with the batholiths is uncertain. The Lady of the Lake Intrusion is a small layered body composed of melagabbro and gabbro that occurs along the south-central margin of the Tobacco Root Batholith near its contact with Archean country rocks. A diorite unit, spatially distinct from the granodiorite/quartz monzonite of the Batholith intrudes the gabbroic rocks of the Lady of the Lake Intrusion. Zircon crystals from the melagabbro and diorite units give U–Pb ages that are very similar to that of the Tobacco Root Batholith at 74.88±0.17 Ma and 76.24±0.08 Ma, respectively. Mineral chemistry, whole rock major and trace element compositions, and oxygen and sulfur isotope ratios have been utilized to evaluate the genetic relation between the Lady of the Lake Intrusion, the diorite, and the Tobacco Root Batholith. No significant variation in the composition of clinopyroxene is observed in different rock units of the Lady of the Lake Intrusion. Minor olivine with Fo64 in the melagabbro unit is interpreted to represent early crystallization in the base of the intrusion. Whole rock major and trace element compositions, as well as results from modeling using the MELTS program, are consistent with the premise that the diorite was produced by fractional crystallization of the same magma that was parental to the gabbros of the Lady of the Lake Intrusion. Both whole rock chemistry and oxygen isotopes support the interpretation that the parental magma was an uncontaminated mantle-derived basaltic magma. In contrast, trace element and oxygen isotopes indicate that the quartz monzonitic and granodioritic rocks of the Tobacco Root Batholith and the gabbroic rocks of the Lady of the Lake Intrusion are not related to the same parental magma. Elevated d18O values of quartz (9–10‰) and feldspar (8–9‰) from the Batholith are consistent with an origin involving partial melting of Archean and Proterozoic country rocks that was initiated by heat supplied by basaltic magma, similar to that which was parental to the Lady of the Lake Intrusion. d34S values of the melagabbro range from 0.1 to 0.7‰, and support the premise that the mantle-derived parental magma was uncontaminated. In contrast, d34S values of the gabbro and diorite units are elevated (0.6 to 6.5‰), and indicate that assimilation of country rock S occurred, primarily via volatile transfer. Sulfur contents of the Lady of the Lake Intrusion are low (<1000 ppm) and no evidence suggests that S was transferred to crustal melts that produced the Tobacco Root Batholith.