Are Cenozoic topaz rhyolites the erupted equivalents of Proterozoic rapakivi granites? Examples from the western United States and Finland

Abstract

Eruptions of topaz rhyolites are a distinctive part of the late Cenozoic magmatic history of western North America. As many as 30 different eruptive centers have been identified in the western United States that range in age from 50 to 0.06 Ma. These rhyolite lavas are characteristically enriched in fluorine (0.2 to 2 wt.% in glass) and lithophile trace elements, such as Be, Li, Rb, Cs, Ga, Y, Nb, and Ta. REE patterns are typically flat with large negative Eu anomalies; negative Nb–Ta anomalies are small or nonexistent; and F/Cl ratios in glasses are high (> 3). These features, together with high Fe/Mg ratios and usually low fO 2, set them apart from subduction-related (I-type) silicic rocks. The rhyolites are metaluminous to only slightly peraluminous, lack indicator minerals of strongly peraluminous magmas, and have low P and B contents; these features set them apart from S-type silicic magmas. Instead, topaz rhyolites have the major and trace element, mineralogic, and isotopic characteristics of aluminous A-type or within-plate granites. Topaz rhyolites were formed during regional extension, lithospheric thinning, and high heat flow. Topaz rhyolites of the western United States crystallized under subsolvus conditions, and have quartz, sanidine, and Na-plagioclase as the principal phenocrysts. Fluorite is a common magmatic accessory, but magmatic topaz occurs only in a few complexes; both are mineralogical indicators of F-enrichment. Many also crystallized at relatively low fO 2 (near QFM) and contain mafic silicate minerals with high Fe/(Fe + Mg) ratios. Some crystallized at higher oxygen fugacities and are dominated by magnetite and have titanite as an accessory mineral. Post-eruption vapor-phase minerals include topaz, garnet, red Fe–Mn-rich beryl, bixbyite, pseudobrookite, and hematite. They are genetically related to deposits of Be, Mo, F, U, and Sn. Topaz rhyolites erupted contemporaneously with a variety of other igneous rocks, but most typically they form bimodal associations with basalt or basaltic andesite and are unrelated to large collapse calderas. In their composition and mineralogy, topaz rhyolites are similar to the evolved members of rapakivi granite complexes, especially those of Proterozoic age in southern Finland. This suggests similarity in origin and lessons learned from these rocks may help us better understand the origins of their more ancient counterparts. For example, all topaz rhyolites in western North America seem to be intrinsically related to extension following a regional period of subduction-related volcanism. Cratonized Precambrian crust is found beneath almost all of them as well. Trace element models, Sr–Nd isotopic data, and geologic associations indicate that topaz rhyolites probably form by fractional crystallization of silicic magma which originated by small degrees of melting of hybridized continental crust containing a significant juvenile mantle component not derived from a subduction zone (i.e., intrusions of within-plate mafic magma). The Sr and Nd isotopic compositions of the topaz rhyolites lie between the fields of contemporaneous mafic magmas and older calc-alkaline dacites and rhyolites. Intraplate mafic magmas and their derivatives appear to have lodged in the crust and were then re-melted by subsequent injections of mafic magma. In turn, the mafic mantle-derived magma probably formed as a result of decompression related to lithospheric extension or to convective-flow driven by the foundering of a subducting lithospheric plate. Although significant uncertainty remains, we suggest that topaz rhyolites (and by extension rapakivi granites) are probably not simply melts of mid-crustal granodiorites, nor are they derived solely from felsic crust that was previously dehydrated or from which melt had been extracted as proposed in earlier papers.