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Petrogenesis of High-K, Calc-Alkaline and Shoshonitic Intrusive Rocks in the Tongling Area, Anhui Province (Eastern China), and Their Tectonic Implications

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Petrogenesis of High-K, Calc-Alkaline and Shoshonitic Intrusive Rocks in the Tongling Area, Anhui Province (Eastern China), and Their Tectonic Implications Petrogenesis of high-K, calc-alkaline and shoshonitic intrusive rocks in the Tongling area, Anhui Province (eastern China), and their tectonic implications Cailai Wu1,†, Shuwen Dong2, Paul T. Robinson1, B. Ronald Frost3, Yuanhong Gao1, Min Lei1, Qilong Chen1, and Haipeng Qin1 1State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 2Chinese Academy of Geological Sciences (CAGS), Beijing 100037, China 3Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82072, USA ABSTRACT ing of differentiated mantle and crustal melts, ing the petro genesis of the two series. Several followed by assimilation–fraction crystalliza- processes have been proposed for the origin The Mesozoic intermediate-silicic intru- tion (AFC) processes. The magmatic activity of these rocks: (1) assimilation of country sive rocks in the Tongling area, Anhui Prov- may have been related to reactivation of the rock by alkaline basaltic magma (Mao, 1990), ince, eastern China, include a high-K, calc- Tongling-Deijiahui structural zone in re- (2) fractional crystallization of lower-crustal alkaline series and a shoshonitic series . Rocks sponse to rapid, highly oblique subduction of melts (Wu, 1986), (3) assimilation of lower- of the calc- alkaline series comprise more the paleo–Pacifi c plate beneath South China. crustal material by alkaline basaltic magma than 90% of the total and consist chiefl y of followed by fractional crystallization (Xing gabbro-diorite , granodiorite, quartz monzo- INTRODUCTION and Xu, 1995), (4) partial melting of basaltic diorite, and porphyritic quartz monzodiorite. lower crust to form tonalitic intrusive rocks These rocks are associated with important The Tongling district, which is situated in the (Zhang et al., 2001), and (5) mixing of mantle- skarn-type copper-iron deposits. They con- eastern part of the Yangtze River basin in derived magmas with those formed by partial tain three types of enclaves: mica-rich vari- Anhui Province, is an ancient copper capital melting of basaltic lower crust (Wang et al., eties that appear to be residues of partially of China and one of the most important metal- 2003). In this paper, we reexamine the origin melted pelitic rock, mafi c quartz monzo- bearing districts in the country (Fig. 1A). The of these granitoids using new sensitive high- diorite, and microdiorite. The shoshonitic polymetallic district is ~40 km long in an E-W reso lution ion microprobe (SHRIMP) U-Pb series consists of pyrox ene monzodiorite, direction and 20 km wide, with a total area of ages, whole-rock geochemistry, and the com- monzonite, and quartz monzonite, which are ~800 km2 (Fig. 1B). Within the district, there positions of the various enclaves hosted in the commonly asso ciated with skarn-type gold are 76 intermediate-silicic intrusive bodies and gran itoids. We review all of the recent data on deposits. Enclaves in these rocks are typi- 54 known ore deposits (Wu et al., 2010a). The the ages of the rocks and their enclaves (X.S. cally pyroxene-rich or amphibole-rich vari- intrusive rocks are mostly intermediate dio- Xu et al., 2004; Du et al., 2004, 2007; Yang eties or amphibole gabbros. Zircon sensitive rites, monzonites, and quartz monzonites that et al., 2007; Zhang et al., 2006; Wu et al., high-resolution ion micro probe (SHRIMP) form an early high-K, calc-alkaline series and a 2001; Wang et al., 2004a, 2004b, 2004c; X.C. U-Pb age data suggest that the granodiorites, slightly later shoshonitic series. The ore depos- Xu et al., 2008), but the validity of some ages is quartz monzo diorites, and gabbro-diorites its are predominantly skarn type with copper, uncertain, because different dating techniques of the calc-alkaline series were generated at iron, and gold mineralization, accompanied by have yielded different ages for the same intru- ca. 146–142, 143, and 140 Ma, respectively. minor strata-bound types in the host carbon- sive body (Zhou et al., 1987;Wu et al., 1996). The shoshonitic rocks range in age from 143 ates (Zhao et al., 1999; Zhai et al., 1992). Some For example, 40Ar-39Ar dating of biotite from to 136 Ma. Although there is some overlap in porphyry-type mineralization is also present, some of the Tongling rocks has yielded ages of reported ages of the two series, contact rela- but it generally occurs only in the deeper parts 140–137 Ma for granodiorite, 137–136 Ma for tions indicate that the shoshonitic rocks post- of the intrusions (Pan and Dong, 1999). The quartz monzodiorite, 138–137 Ma for pyroxene date the calc-alkaline varieties. On the basis total reserves in this district have been esti- monzodiorite, and 134 Ma for gabbro-diorite of the geochemistry of the two series and the mated to be 500 Mt copper and 150 t gold (Wu (Wu et al., 1996, 2001), but these ages only character of their enclaves, the shoshonitic et al., 2010a). record the time at which the intrusive bod- series is thought to have formed primarily by Because of their associated copper and ies cooled through ~300 °C, the Ar-Ar clo- differentiation of a mantle-derived, weakly gold deposits, the intrusive rocks have been sure temperature of biotite (Cliff, 1985). A contaminated, alkali basalt magma, whereas studied for many years (e.g., Chang and Liu, few zircon SHRIMP U-Pb ages have recently the high-K, calc-alkaline series refl ects mix- 1983; Tang et al., 1998; Xing and Xu, 1995, become available (Yang et al., 2008; X.C. Xu 1996; Zhou et al., 1993; Wu et al., 1996, et al., 2008), but these studies all focused on †E-mail: [email protected] 2000, 2003), but there is no agreement regard- individual bodies . Here, we report new zircon GSA Bulletin; January/February 2014; v. 126; no. 1/2; p. 78–102; doi: 10.1130/B30613.1; 12 fi gures; 10 tables. 78 For permission to copy, contact [email protected] © 2013 Geological Society of America Petrogenesis of intermediate-acid intrusive rocks and zircon SHRIMP U-Pb dating in the Tongling area, Anhui Province (eastern China) A B Figure 1. Geological sketch map of Tongling area, Anhui, China. R—Tertiary system; K2, K1—Upper and Lower Cretaceous 2 3 1 system; J3—Upper Jurassic system; J1–2—Middle and Lower Jurassic system; T2 -T2 —Middle Triassic system; D3-T2 —Upper Devonian system–Middle and Lower Triassic system; S—Silurian system; «—dating sample locations. Geological Society of America Bulletin, January/February 2014 79 Wu et al. SHRIMP ages for four plutons and discuss the Paleozoic and Triassic sedimentary rocks form LITHOLOGY OF THE INTRUSIVE petrogenesis of the intrusive rocks and their a series of complex, NE-trending folds with ROCKS AND THEIR ENCLAVES enclaves, as well as the tectonic environment thrust faults along their limbs (Zhai et al., in which they formed. 1996), many of which are cut by NW-trending The high-K, calc-alkaline series rocks range brittle faults (Fig. 1B). Several possible base- in composition from gabbro-diorite through GEOLOGICAL SETTING ment faults, including the major Tongling- quartz monzodiorite and granodiorite to aplitic Deijiahui structural zone, have been identifi ed granite. Most of these rocks have hypidiomor- The continental core of China is composed from geophysical data, and their intersections phic granular textures, except for the gabbro- of the South China block and the North China are thought to partially control the location diorites, which have gabbroic-diabasic textures, craton, which were welded together along the of the Mesozoic intrusions (Ren et al., 1992; and some granodiorites with porphyritic tex- Dabie-Sulu orogenic belt between ca. 250 and Zhai et al., 1996; Chang et al., 1996; Tang tures. The rock-forming minerals are plagio- 220 Ma. The South China block was formed et al., 2004). clase, quartz, amphibole, biotite, and potassium by collision and amalgamation of the Yangtze Seventy-six individual intrusions have been feldspar, most of which range from 1.2 mm to and Cathaysian blocks at ca. 880 Ma. The identifi ed in the Tongling area (Fig. 1B). Most 2.2 mm in size. Some porphyritic granodiorites Tongling area lies in the Yangtze polymetal- of these are small stocks and dikes, gener- have plagioclase phenocrysts up to ~6 mm. lic belt located in the northeastern part of ally with outcrop areas of 0.05–3 km2, locally Many of the plagioclase grains have diffuse the Yangtze block (Chang et al., 1991; S. Xu accompanied by small sills, apophyses, and cores crowded with opaque inclusions; others et al., 1992; Pan and Dong, 1999) (Fig. 1A). veins. They are hosted mostly in Silurian to contain inclusions of apatite. Some alkali feld- It is nearly perpendicular to the Tan-Lu fault, a Triassic carbonates and quartz sandstones, and spar phenocrysts are rimmed by plagioclase, major left-lateral, strike-slip fault that offsets less commonly in siliceous rocks. The intrusive suggesting magma mixing (cf. Hibbard, 1991). the Dabie ultrahigh-pressure (UHP) metamor- rocks are divided into a high-K, calc-alkaline These rocks contain three types of enclaves, phic belt several hundred kilometers to the series and a shoshonitic series on the basis of which are different from those in the shoshonitic northeast (Fig. 1A). The exact age of initiation their petrochemistry. series. Their main features are: of the Tan-Lu fault is uncertain, but 40Ar-39Ar The high-K, calc-alkaline rocks occur as (1) Mica-rich enclaves: These enclaves occur dating of amphibole in ductile shear zones in NE-trending stocks in Carboniferous dolomitic mainly in the granodiorites. They are black, the southern part of the fault suggests it was limestone and Permian to Triassic carbon- elliptical bodies that range from 4 to 8 cm in activated no later than 143 Ma, in response to ate rocks.
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