Fuwei Xie

Late Cretaceous magmatic activity in the southern Lhasa terrane: insights from the Dazhuqu hornblende gabbro and the Xietongmen granite porphyry

ABSTRACT The mechanisms triggering the emplacement of Cretaceous magmatic rocks related to Neo-Tethyan subduction in the southern Lhasa subterrane (SLT) remain controversial because geochronological, geochemical, and isotopic data from the Cretaceous magmatic rocks in the western portion of the SLT are lacking. This paper provides the first report indicating that the Dazhuqu hornblende gabbro and Xietongmen granite porphyry were generated in the Late Cretaceous (ca. 100 Ma and ca. 68 Ma, respectively) in the western portion of the SLT. The Dazhuqu hornblende gabbro is characterized by high MgO, Na2O, Cr, Co, and Mg# values, enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and depletion in high field strength elements with no Eu anomalies. The Xietongmen granite porphyry displays high SiO2, low MgO, Mg#, Cr, Co, and Ni, enrichment in LREEs and LILEs, flat heavy rare earth element (HREE) patterns and negative Eu anomalies. Zircons from the Dazhuqu hornblende gabbro and Xietongmen granite porphyry display high positive εHf(t) ranges from 11.79 to 14.91 and from 9.05 to 12.38, respectively. The Dazhuqu hornblende gabbro was produced by the partial melting of 12–15% garnet peridotite in the depleted mantle that was metasomatized by fluids released from the dehydrating subducted Neo-Tethyan oceanic slab. This magma was then emplaced at an upper crustal depth of 5.3–7.0 km. The Xietongmen granite porphyry was generated by the partial melting of underplated basaltic crust during the subduction of the Neo-Tethyan oceanic crust and formed in the presence of pyroxene, plagioclase, and minor hornblende in its magmatic source. In combination with previously published data from Cretaceous magmatic rocks in the SLT, our data suggest that the petrogenesis of the Dazhuqu hornblende gabbro was related to flat or low-angle slab subduction of the Neo-Tethyan oceanic crust during 109–97 Ma and that the formation of the Xietongmen granite porphyry was related to lithospheric delamination in the western portion of the SLT after ca. 68 Ma. Graphical Abstract

The Genetic Types of Mineral Deposits of Xiongcun District, Gangdese Porphyry Copper Belt, Tibet, PRC

mineralization in the Gangdese porphyry copper belt (GPCB),located in the middle of the GPCB. Its southern margin is Xigaze forearc basin (Lang, et al., 2012a). Three main Cu-Au deposits (No.I, No.II, No.III deposit) and many mineralized bodies have been found in this district. Three main Cu-Au deposits are NW-striking. Dongga gold deposit is located about 2 km northwest of No.I deposit and 0.5 km southwest of No.II deposit. It is the first gold deposit have been found in the GPCB. The genetic type of mineral deposits in Xiongcun district has a large controversy in the last decade. In the early exploration stage, understanding of the genesis of No.I deposit mainly have the following kinds: epithermal deposit (Ding,2004), porphyry deposit (Lang et al.,2007) and porphyry-epithermal deposit (Xu et al.,2006). Along with the large-scale exploration work, Lang (2007) and Tang et al. (2010) established this deposit is a porphyry copper-gold deposit,which has been widely accepted. In island arc porphyry copper-gold deposit theory as the guidance, comprehensive information of geology, geophysics, geochemistry and remote sension, we gradually found No.II, No.III deposit and many mineralized bodies in the peripheral areas. mineralization characteristics of the No.II, No.III copper gold deposits are similar, but have significant difference compared with No.I deposit. Lang (2012b)for the first time put forward that No.I deposit is reduced porphyry copper-gold deposit (RPCD) and No.II, No.III deposits are oxidized porphyry copper-gold deposit (OPCD). In addition, there are mainly two kinds of views about the genetic type of Dongga gold deposit: meso-epithermal structure altered rock type gold deposit(Xing et al., 2003)and epithermal gold deposit (Cao et al.,1996;Lang, 2007). Deposit genetic types have many disputes because insufficient understanding of metallogenic features. There are at least two epoches of porphyry copper-gold mineralization in Xiongcun district and typical representative is No.I, No.II deposit. At No.I deposit, predominant ilmenite over magnetite suggest that the granitoids related to mineralization are ilmenite-series Itype granitoids (Rowins et al., 2000; cao et al., 2014). Rowins (2000) argues that RPCD is closely related with ilmenite-series I-type granitoids. The No.I deposit is characterized by widely developed pyrrhotite and ilmenite, which have similar mineral assemblages to the RPCDs, such as 17 mile Hill, San Anton, Madeleine (Rowins et al., 1997;Randall et al., 1994;Ague and Brimhall, 1988). Xu et al. (2006) have indicated that the ore-forming fluid of No.I deposit is a immiscible CO2-CH4-N2 system. Takagi and Tsukimura (1997) suggested when the magma is buffered by CH4-CO2,no magnetie will be formed throughout the course of crystallization. The No.II deposit is characterized by widely developed magnetite and anhydrite, which have similar mineral assemblages to the OPCDs (Rowins,2000). The shallow gold mineralization of Dongga gold deposit controlled by northwest fracture systems and deep mineralization controlled by hidden explosive breccia in the contact alteration zone (Cao et al., 1996). The mineralization is closely related to silicification and sericitization. The silicification locally manifested as quartz-sulfide vein. Characteristics of fluid inclusions suggest that the mineralization temperature and pressure are low (Ding,2004;Cao et al., 1996). The ore-forming fluid is given priority to with atmospheric precipitation (Lang,2007). The above characteristics indicate that the deposit is similar to typical epithermal gold deposit. In recent decades, many scholars study epithermal deposit related to porphyry deposits metallogenic system(Eaton et al., 1993;Hedenquist et al., 1998;Sillitoe et al., 1999; Sillitoe et al., 2003;Richards et al., 2006). Abundant pyrrhotite have been found at Dongga gold deposit. Gaseous composition of ore-forming fluid also appear a XIE Fuwei,TANG Juxing and LANG Xinghai, 2014. The Genetic Types of Mineral Deposits of Xiongcun District, Gangdese Porphyry Copper Belt, Tibet, PRC. Acta Geologica Sinica (English Edition), 88(supp. 2): 629-630.