Origin and petrogenetic implications of garnet from Rb-rich pegmatites in North Qinling Orogen, China

A group of Rb-rich granitic pegmatites near Ziyugou, Danfeng, in the North Qinling Orogen lack typical lithological zoning and have a simple mineralogy dominated by quartz+microcline+albite with garnet in minor quantities. Biotite is rare, and muscovite was mostly formed during greisenization. Graphic texture can be observed within the pegmatites near the contact with the country rock. The pegmatites have localized garnet-rich zones that contain 5-15 vol.% garnet and accessory zircon, columbite, xenotime, and thorite. The pegmatites are peraluminous (A/CNK=similar to 1.02-1.23) and contain low Ca but high alkali and elevated Rb (similar to 600-1300ppm). In REE patterns and spider diagrams, the pegmatites exhibit a pronounced negative Eu anomaly, strong Ti, P, Sr, and Ba depletion, and enrichment of Rb, HFSE, and HREE. In addition, the garnet-rich zones contain lower K and Rb but higher Na, Mn, Fe, Zr, Hf, Nb, Y, Th, and U concentrations than the part with graphic texture and accessory garnet. Two types of garnets occur in the pegmatites. Type 1 garnet is well-zoned and closely associated with the above-mentioned HFSE minerals in the garnet-rich zones, whereas Type 2 garnet included in either feldspar or quartz defining graphic texture has no associated HFSE minerals. Both types of garnets are almandine-spessartine solid solution and contain trace to minor amounts of pyrope, grossular, and uvarolite. Based on garnet chemistry, Type 1 garnet can be subdivided into two subtypes, namely, Type 1a with higher Alm content and Type 1b with higher Sps and Prp contents. Unlike garnets from LCT-type pegmatites but similar to those from NYF-type pegmatites, the garnets from the Ziyugou pegmatites contain up to 1.12wt.% Y2O3 and 1.16wt.% Yb2O3, which makes garnet the main carrier of Y and HREE. This explains why the garnet-rich part contains higher Y and HREE. Type 1 garnet has higher Mn/Fe ratio than Type 2 garnet, indicating that the garnet-rich part of the pegmatites likely represents a more evolved melt. Although the pegmatites somehow show S-type granite affinity (i.e., elevated Rb concentration and strong depletion in Sr and Ba), the HFSE and HREE enrichment of the pegmatites and garnet chemistry suggest that the pegmatites may have had a hybridized source.