An arc-related Phanerozoic anorthosite?: the petrogenesis of the Indian Rock Anorthosite in the Sierra Nevada Batholith

Abstract

Massif-type anorthosites are batholithic bodies consisting primarily of plagioclase and minor mafic phases. Most importantly, they are known to be temporally restricted to the Proterozoic Eon (2.643 and 0.502 Ga). Several different tectonic settings for the massif-type anorthosite have been suggested (i.e. continental rift, plumes, catastrophic magmatic events), although recent studies suggest a more likely tectonic setting in a continental arc environment. However, no anorthositic bodies with both massiftype characteristics and cognate anorthosite inclusion have been found in Phanerozoic arc environments, such as the Andes, casting doubt on the continental arc hypothesis. In this project, we focus on a small body of anorthosite known as the Indian Rock Anorthosite (IRA), situated on the eastern flank of the Mesozoic Sierra Nevada Batholith. Although this body has been mapped by the USGS, almost no scientific data exist on the petrogenesis of the IRA. Investigating why this body exists in the subduction-related Mesozoic Sierra Nevada Batholith is crucial to not only understand the temporal restriction and tectonic setting of massif-type anorthosites but to also question if and how the subduction environment may have changed over the Proterozoic-Phanerozoic transition. The IRA, an approximately ~1 km2 body, is found near the headwaters of Thibaut Creek on the eastern scarp of the Sierra Nevada Mountain range and is surrounded by mafic granodiorites of the Black Canyon Pluton and the McDoogle Pluton. The IRA is light grey and has an average grain size of about 4 mm. Over 90 % of the rock consists of euhedral plagioclase with an average composition of An84 and the remaining 10 % is made up of clinopyroxene (locally altered to hornblende and chlorite), with quartz and K-feldspar occurring as interstitial phases. Two samples from the IRA (sample IRA 4-2 and IRA 18-6) yield zircon U-Pb ages of 94.01 ± 0.05 and 95.29 ± 0.06 Ma, respectively, which rules out the possibility of the IRA being a fragment of an older Proterozoic crust trapped in a Mesozoic-dated arc. The surrounding granodiorites of the Black Canyon Pluton and the McDoogle Pluton were dated at 96 ± 2 Ma and 94 ± 4 Ma, respectively. The IRA shows similarities in geochemistry with its immediate surrounding rocks in terms of ƐNd (95Ma) and initial 87Sr/86Sr (95Ma), indicating that they are coeval and comagmatic. In its field structure and plutonic nature, the IRA resembles a typical, albeit small, massif-type anorthosite. However, the high An content in the IRA (anaylsed: An72 - An89, calculated: An79 - An82) is more typical of Archaean anorthosites (An61- An94) and cognate anorthosite inclusions in arcs (An63- An100) (Kristmannsdóttir, 1971; Ashwal, 2010; Ashwal and Bybee, 2017). The adcumulus texture, mineral content and whole-rock geochemistry resembles are cognate anorthosite inclusions in continental arcs. The major differences between the IRA and Proterozoic massif-type anorthosites in general, besides the age, is its small size, insignificant positive Eu anomalies and the high An content in plagioclase. The magnitude of Eu anomalies and the changes in An content are a function of pressure, temperature, water content and oxygen fugacity during the formation of magmatic rocks. AlphaMELTS-based thermodynamic models show that conditions of formation of the IRA include a combination of hydrous magmas (H2O = 0.5 wt%) at relatively high temperatures and pressures (~1213 ◦C and <10 kbars) and under oxidising conditions. These conditions are ideal for the production of trivial amount of plagioclase production with less pronounced Eu anomalies due to plagioclase fractionation and increased An content. AlphaMELTS modelling also established that the IRA formed from a hydrous high-Al magma (Al2O3 >17 %) (tholeiitic basalt) that was contaminated by granitic crustal material. These petrogenetic characteristics are typical of young Phanerozoic subduction zones, where the colder subducting slab is capable of transporting water deeper in the crust under high pressure and high temperatures, typified by Alpine or blueschist/eclogite metamorphic facies. The IRA is unique in a sense that it occurs in a Phanerozoic continental arc setting and it relates to both cognate anorthosite in continental arcs in term of its adcumulus texture, mineral content and also shows similar plutonic nature of a typical massif-type anorthosite. The discovery of the IRA and the description of its petrogenesis in this work haves shown that bodies of anorthosite, reminiscent of massif-type, can form in Phanerozoic arc environments. This is significant for the petrogenesis of anorthosites in general and our understanding of the ~2.7 - 0.5 Ga temporal restriction of massif-type anorthosites, which have been hypothesized to form in continental arc environments. The paucity of massif-type anorthosites in Phanerozoic arcs need no longer disprove the continental arc hypothesis, as we have shown that such types of anorthosite, albeit as smaller bodies with some important chemical differences, can form in these environments. In addition, we can also use the presence of the IRA to understand changes that may have occurred in continental arc environments from the Proterozoic to Phanerozoic Eons. Various published temporal datasets along with numerical modelling have suggested that Proterozoic subduction zones were dry, producing magma at low pressure and high temperatures with relatively low oxygen fugacity, which allowed for the vast accumulation of plagioclase with large Eu anomalies and intermediate An contents. In contrast, young Phanerozoic subduction zones are hydrated, occurring at high pressure and high temperature, under oxidising conditions. These conditions will reduce the production of plagioclase in the differentiating magmas derived from the mantle wedge and increase the An content of this plagioclase. This would explain why so few discrete bodies of anorthosite are found in Phanerozoic arcs and where they are found as discrete bodies (e.g. the Indian Rock Anorthosite) or as cognate inclusions in arc volcanics, the An content ranges from 70 – 100 %. These results suggest that fundamental changes in continental arc dynamics between the Proterozoic and Phanerozoic may be responsible for the temporal restriction of Proterozoic anorthosites.