The composition and petrogenesis of the lower crust: A xenolith study

Publisher Website
Google Scholar
Abstract
Granulite facies lower crustal xenoliths from a single basaltic vent (Hill 32) in the McBride volcanic province, north Queensland, Australia, illustrate the extreme lithologic diversity of the deep crust. These xenoliths are dominantly mafic, but intermediate and felsic granulites represent a significant proportion (∼20%) of the xenolith population. All xenoliths have high‐grade mineralogies and generally well‐equilibrated textures, as well as decompression features which are indicative of derivation from deep crustal levels (0.7–1.0 GPa). Major and trace element chemistry for 12 xenoliths, chosen to span the observed lithologic range, are used to constrain the petrogenesis of these rocks. In the smaller, layered samples, metamorphic differentiation may lead to nonrepresentative sampling. However, in most instances such secondary processes can be identified and the original chemical characteristics delineated. The mafic xenoliths formed through a variety of processes, including crystallization of mafic magma, crystal accumulation from mafic and felsic magmas, and partial melting of intermediate compositions to yield a mafic residuum. The two analyzed intermediate xenoliths are metasediments, based upon high alumina to alkali ratios and rare earth element patterns, whereas the two felsic xenoliths have compositions similar to igneous rocks. Comparisons of noncumulate and/or restite xenoliths with unmetamorphosed rock types show that K and Rb are variably depleted whereas Th and U are strongly depleted, in all rock types. The large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains, suggesting similar processes were operative. Suggestions of an anomalously Ba‐ and Sr‐rich lower crust are not supported by the data. By using the observed lithologic proportions of xenoliths at Hill 32, and the chemical analyses presented here, a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions. The weighted mean composition is mafic, and formed through a combination of basaltic underplating, crystal accumulation, tectonic underplating, and partial melting. No simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in Phanerozoic continental margin settings.

This publication has 56 references indexed in Scilit: