No abstract

No abstract

Single zircon LA-ICP-MS U-Pb dating and lithogeochemical studies have been performed on the Xianggou monzonitic granitic porphyry outcropped in the Ma'anqiao gold deposit. A weighted average U-Pb age of 242.0±0.8 Ma for Xianggou monzonitic porphyry has been obtained. This corresponds with the conclusions of previous studies indicating a syn-orogenic age (242±21 Ma) of the Qinling Orogenic Belt, suggesting that the formation of the Xianggou granite should be associated with the collisional event of the North China Plate and the Yangtze Plate in the Indosinian period. The Xianggou granite is characterized by the high silicon and alkali of high K calc-alkaline series granites. It is rich in Al (Al 2 O 3 =14.49%-15.61%) and Sr (457.10-630.82 ppm), poor in Y (<16 ppm) and HREE (Yb<0.45 ppm), and exhibits high ratios of Sr/Y (76.24-97.34) and (La/Yb) N (29.65-46.10), as well as strongly fractionated REE patterns. These geochemical characteristics suggest the Xianggou granite can be classified as C-type adakitic rock. The initial Sr isotope ratios for the Xianggou granite vary from 0.70642 to 0.70668, ε Nd (t) values from −4.54 to −3.98, and T DM values from 1152 Ma to 1220 Ma. The low ε Nd (t) and I Sr and high T DM values, as well as Na 2 O/K 2 O ratios of the Xianggou granite are close to 1 (Na 2 O/K 2 O=0.95-1.10), indicating that it is not an I-type adakite formed by partial melting of the subducting oceanic crust, nor adakitic rock formed by melting of the underplated basaltic lower crust, but the product of partial melting of the nonunderplated basaltic thickened lower crust. Zircons from the Xianggou pluton have a homogeneous Hf isotopic composition with negative ε Hf (t) values (between −9.7 and −5.9, with an average of −6.9), indicating that the rock-forming materials were mostly extracted from the ancient crust, not from the depleted mantle. The Xianggou monzonitic granitic porphyry is rich in LILE and LREE and depleted in HSFE, HREE and Y; the composition of trace element and REE are similar to those of the syn-collisional granites. The geological and geochemical characteristics of the Xianggou granite reveal that it was a product of partial melting of the basaltic rocks from the thickened lower crust, triggered by continental collision, which occurred in the geodynamic background of continental-continental collision and shearing within the crust. The Xianggou granite was intruded in the compressive orogenic environment 242 Ma ago, but the gold mineralization occurred in the transitional environment of compression to extension around 170 Ma ago, lagging behind the intrusive age of the Xianggou granite by about 70 Ma. Meanwhile, the distribution of trace elements and REEs of the Xianggou monzonitic granitic porphyry is distinct from that of ores, suggesting the absence of direct genetic relationship between the Xianggou granite and gold mineralization. In contrast, the relatively high ore-forming elemental content of the Xianggou monzonitic granitic porphyry is due to the rock having experienced Au-bearing hy...

The Zhongchuan district is an important component of the metallogenic belt in the Western Qinling. The Zhongchuan granite pluton occurring in the centre of the Zhongchuan metallogenic area has been poorly constrained, though the Triassic granite in Western Qinling has been well documented. In‐situ zircon U–Pb ages, Hf isotopic compositions and whole‐rock geochemical data are presented for host granite and mafic microgranular enclaves (MMES) from the Zhongchuan pluton, in order to constrain its sources, petrogenesis and tectonic setting of the pluton. The distribution of major, trace and rare earth elements apparently reflect exchange between the MMES and the host granitic rocks mainly due to interactions between coeval felsic host magma and mafic magma. The zircon U–Pb age of host granite (231.6 ± 1.5 to 235.8 ± 2.3 Ma) has overlapping uncertainty with that of the MMES (236.6 ± 1.3 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the MMES (εHf(t) = −13.4 to 4.0) is distinct from the host granite (εHf(t) = −15.7 to 0.0), indicating that both enriched subcontinental lithosphere mantle (SCLM) and crustal sources contributed to their origin. The zircons have two‐stage Hf model ages of 1064 to 1798 Ma for the host granite and 858 to 1747 Ma for the MMES. This suggests that the granitic pluton was likely derived from partial melting of a Late Mesoproterozoic crust, with subsequent interaction with the SCLM‐derived mafic magmas in tectonic affinity to the South China Block. Copyright © 2012 John Wiley & Sons, Ltd.

No abstract

No abstract

No abstract

No abstract