The Petrogenesis of the Late Cretaceous Mamba Pluton from the Eastern Gangdese: Constraints from Mineralogy

Occurred as stock,the late Cretaceous Mamba pluton, located at the southeast part of the Gangdese terrane, was one of the granitoid intrusions in the Mesozoic-Cenozoic Gangdese gaint magmatic belt, which is composed of Southern Gangdese, Gangdese back-arc fault uplift belt, Middle Gangdes, The Shiquanhe-Laguo Tso-Yongzhu-Nam Tso-Jiali Ophiolitic Méange Zone, and northern Gangdese from south to north, bounded by Indus-Yarlung Zangbo suture zone to the south, and Bangong Tso-Nujiang suture zone to the north(Zhu et al., 2008a), and the Mamba Pluton intruded pre-Ordovician Sumdo Group and early formed intrusions(T3, J1, J3), was unconformably overlied by Eocene Pana Formation. The modal mineralogy of selected samples from Mamba Pluton are calculated via Matlab: quartz(25%), plagioclase(40%), K-feldspar(18%), biotite(9%) and hornblende(5%), with subordinate primary epidote(<1%), and the accessory mineral phases include magnetite(1%), apatite(<1%),sphene(<1%), etc, corresponding to mega-porphyritic hornblende-biotite granodiorite. Hornblende and Biotite are the most common ferromagnesian phases in Mamba Pluton, which are chemically and structurally sensitive to prevailing physico-chemical conditions of host magmas(Czamanske and Wones, 1973; Czamanske et al., 1977; Jiang and An, 1984; Kumar and Pathak, 2010; Speer, 1987a, b; Wones and Eugster, 1965; Zhou, 1986). Specifically, Hornblende occurs as euhedral(rhombic) to subhedral crystal, composing c.a. 5% of the specimens, and depending on the samples, partially replaced by epidote, and some show simple twinnings. Biotite is euhedral to subhedral, and some of them occurs intergranular, moreover, Goethite occurs as weathering products in some biotite. Besides, the texture relationship indicates that its predated hornblende, i.e. hornblende encompasses biotite. Essentially, the K-feldspar is microcline in terms of classification. The X-ray powder diffraction analyses of K-feldspar megacrysts from Mamba Pluton provide degrees of monoclinic ordering 0.665-0.737 and triclinities of 0.394-0.515. Equilibrium temperatures of K-feldspar vary from 565 to 594 Celsius via the method of (Ma, 1988), additionally, the proportion of Or% range from 93.01 to 95.23. Most of biotite samples from Mamba Pluton fall between the NNO and HM buffers, but very close to NNO buffer, indicationg relatively moderate oxdizing conditions.. Emplacement depths estimated from aluminum-in-hornblende geobarometry indicate that the Mamba Pluton was emplaced at the pressure of c.a.1.33kbar, corresponding to 4.75km in depth.Temperatures of emplacement calculated with the hornblende-plagioclase thermometer range from 645 Celsius to 780 Celsius.Since the Mamba Pluton and contemporary plutons are unconformably overlain by Eocene volcanic rocks, namley Pana Formation regionally, thus the Mamba pluton must have been exposed at(or near) the surface prior to the Eocene, indicating the existence of protoplateau as proposed by Wang et al(2008).