The peralkaline Mount Rosa granite: contrasting mineralogy and geochemistry observed in the Mount Rosa granite, Pikes Peak batholith, Colorado

Abstract

The Mount Rosa complex is a sodic igneous pluton within the ~1.08 Ga Pikes Peak batholith, a typical A-type granite located in the Colorado Front Range. Diverse lithologies are associated with emplacement of the complex; these included peraluminous to peralkaline granitic rocks with several associated minor rock types, such as various dikes and pegmatites. Recent geologic and geochemical studies of the complex revealed the Mount Rosa Granite to have a complex petrogenesis within the pluton. This granite is host to complex Ti-bearing minerals, astrophyllite [K3Fe2+7Ti2Si8O26(OH)5] and aenigmatite [Na2Fe2+5TiSi6O20], which are noted to represent highly peralkaline rocks on the evolutionary path from miaskitic to agpaitic compositions. Researchers have been studying agpaitic rocks to decipher the magmatic processes which drive the transition toward the highly evolved melts and rare metal deposits because of their enrichment of high field strength elements (HFSE). Further understanding of this process may allow for more efficient, targeted exploration in potentially economic alkaline complexes. Studying the Mount Rosa Granite provides a unique opportunity to observe the transitional agpaitic stage of melt evolution in a peralkaline granite as the occurrences of complex HFSE-bearing titanosilicates differ. The peralkaline Mount Rosa Granite is a widely homogeneous, sodic amphibole granite with minor amounts of astrophyllite that occurs in radial clusters. The amphibole in the main body of the Mount Rosa Granite occurs as oikocrysts, enclosing previously formed phenocrysts of quartz and alkali feldspar, and forming late in the crystallization sequence. The oikocrystic amphibole granite is interpreted to be representative of the majority of the peralkaline melt, in which saturation of water was reached late in the crystallization sequence prompting late amphibole formation. At the margins of the Mount Rosa Granite in contact with the Pikes Peak Granite, more variable textural features are observed over a small area, including pegmatitic granite, flow banding, and astrophyllite pockets. This texturally heterogeneous region shows areas of prismatic amphibole granite, where amphibole occurs as prismatic phenocrysts forming due to water saturation early in the crystallization sequence and is observed as unaligned grains to strongly aligned grains in flow bands. Type I pegmatites in the area are interpreted as local coarse-grained pegmatitic Mount Rosa Granite which contain observed occurrences of aenigmatite. Amphibole of three different compositions are identified in the Mount Rosa Granite; katophorite-richterite in a Mount Rosa granitic dike, riebeckite in the pegmatitic granite, and arfvedsonite in the oikocrystic and prismatic amphibole granites. With increasing alkalinity [(Na+K)/Al] of the host rock, amphiboles generally transition from katophorite-richterite (sodic-calcic) to riebeckite and arfvedsonite (sodic) with magmatic differentiation illustrated by the exchange vector Aliv+Ca=Si+(Na, K). In correlating amphibole compositions and textures with whole rock geochemical data, oikocrystic arfvedsonite occurs in samples with a relatively consistent alkalinity index of ~1.25. In contrast, prismatic arfvedsonite occurs in samples with a variable alkalinity index ranging from ~1.15 to 1.35. The alkalinity index of samples, interpreted to represent the degree of melt differentiation, indicates that local areas of prismatic amphibole granite crystallized at variable degrees of melt differentiation while oikocrystic amphibole granite was more homogeneous and consistent in melt evolution. The most alkaline samples were collected from a discrete melt pocket of prismatic amphibole granite which contains both astrophyllite and aenigmatite. These melt pockets may be interpreted as the most evolved portions of the peralkaline melt. The main body of the Mount Rosa Granite contains minor abundances of astrophyllite and zircon, a transitional agpaitic mineral assemblage. Discrete pockets and pegmatitic zones of granite are also host to transitional agpaitic mineral assemblages, yet the abundance of astrophyllite in the pockets and the occurrence of aenigmatite, a more rare titanosilicate, are interpreted to indicate a more evolved composition than the majority of the Mount Rosa Granite. The Mount Rosa granitic dike, viewed as a link between the peraluminous Pikes Peak Granite and the peralkaline Mount Rosa Granite, hosts the miaskitic mineral assemblage of ilmenite and zircon. The interpretation of the mineral assemblages, mineral compositional trends, and textural varieties of granite suggests that the Mount Rosa complex is a composite pluton where rock types range from less evolved, miaskitic compositions to more evolved, agpaitic compositions. The granitic dikes in the complex likely form from a miaskitic melt, the least evolved of peralkaline melt compositions. The large magma chamber of the peralkaline Mount Rosa Granite is proposed to have been a transitional agpaitic magma with oikocrystic. Subsequent less voluminous pulses of magma along the margins of the magma chamber are slightly more evolved containing prismatic amphibole. These prismatic amphiboles are subject to magmatic flow differentiation influencing alignment and accumulation of grains and the formation of residual melt pockets. The localized melt pockets near the margin of the Mount Rosa Granite magma chamber contain complex titanosilicates in greater abundances suggesting melt compositions more evolved than the transitional agpaitic magma in the complex. Despite the occurrence of zircon rather than other complex zirconosilicates, local astrophyllite pockets are considered to have been formed from agpaitic melts, the most evolved melts before the formation of complex NYF pegmatites in the complex.