Is a magma of subalkaline monzo-granodiorite series parental to Late Jurassic calc-alkaline granites of the Eastern Transbaykaliya and associated rare-metal mineralization?
Institute of Experimental Mineralogy RAS, Chernogolovka, Russia, Zaraisky@iem.ac.ru
The Kukulbey complex (J3) of rare-metal granites in Eastern Transbaikalia belong to postorogenic reactivation calc-alkali granite-leucogranite series. Hypabyssal intrusions of these granites are checked deep seated faults which crossing thickness of earlier consolidated regions. At the watershed between the Turga and Borzya rivers Kukulbey granite massifs extend for 110 km in the NE–SW direction in the following order: Belukha, Bukuka, Turga, Achikan, Etyka, Oldonda, Soktuy, Sherlovaya Gora, Adun-Chelon, and Ary-Bulak massifs. On this territory, sometimes within one and same massifs, are present granites several more early monzo-granodiorite Shakhtama complex (J2-3). Majority of the researchers links origin of Kukulbey granites mainly with Continental Crust source, but Shakhtama serits with the Mantle participation (Kovalenko et al., 1999; Troshin, 1985; Syritso, 2002).
Together with that question about a role of the Crust-Mantle interaction in origin of the Kukulbey granite magma and, especially in genesis lithium-fluorine granites and connected with them rare-metal mineralization remains rather discussion. The Kukulbey granites are characterized by a uniquely high degree of fractionation of the parental granitic magma, forming three intrusive phases. The biotite granites of phase 1 are barren, the leucogranites of phase 2 are accompanied by greisen Sn-W mineral deposits (Spokoininskoe and others), and the final dome-shaped stocks of amazonite Li–F granites of phase 3 host Ta deposits of the “apogranite” type: Orlovka, Etyka, and Achikan. All granitic rocks of the complex are roughly simultaneous and have an age of 142 0.6 Ma (Kostitsyn, 2004). The rare-metal specification of Kukulbey complex in Shachtama granite is absent. With broadly wide-spread in Transbaykalia monzo-granodiorite (latite) series are usually associated the Mo, Pb-Zn and Au mineralization. However, uncertainty available data allows expect different degree of the Shakhtama latite magma participation in genesis Kukulbey granite-leucogranite complex and associated with him rare-metal mineralization (Sn, W, Be, Ta, Nb,Li). This promotes the small time gap between Shachtama and Kukulbey complexes (near 10 Ma), close weakly negative values εNd, and practically alike 87Sr/86Sr ratio at a rate of 0.705-0.708 (Kostitsyn et al., 2004; Syritso et al., 2007). Different variants participation of the mantle latite magmas are proposed for genesis of Kukulbey rare-metal mineralization: from as energetic factor (only as source of the heat) up to suggestion of crucial role of mantle fluid in the melting of Crust substrate and even up to suggestion that latite magma is the paternal source for Kukulbey granites (Kovalenko et al., 1999; Syritso, 2002; Syritso et al 2007; Kozlov and Efremov, 1999; Troshin, 1985).
All of the intrusive massifs, stocks, and dikes of rocks of the Kukulbey and Shakhtama complexes
were sampled during our fieldwork in the Kukulbey and Khangilay ore areas (1995-2005. The samples were analyzed for major elements and F by conventional “wet” chemical techniques and by XRF. Minor and trace elements were determined by ICP/MS-AES. The analytical database includes near 300 samples (Zaraisky et al., 2009). Figures 1 and 2 presents a SiO2–Zr/Hf diagrams for the rocks of Shachtama and Kukulbey complexes with the evolutionary trends of the crystallization fractionation (Zaraisky et al., 2009). In the diagrams Fig.1 the granitoids of the Shakhtama complex,which do not bear elevated concentrations of rare metals, display a well pronounced trend of usual crystallization differentiation due to an increase in the content of silica according to a decrease in the (FeO + MgO +CaO) contents in the course of melt crystallization. The Zr/Hf ratio systematically decreases from 45 in diorite to 25 in granite. The Shakhtama complex contains no leucocratic granites. This trend is weakly pronounced in the granites of the Kukulbey complex because the original composition of the granites was close to the eutectic (Fig. 2). Their differentiation trend is thus nearly vertical and almost perpendicular to the differentiation trend of the Shakhtama rocks. A decrease in the Zr/Hf ratio generally points to a systematic fractionation sequence: the Zr/Hf ratio of the rocks systematically decreases from intrusive phase 1 (40–25) to phases 2 (20–30) and 3 (10–2).
The Kukulbey Li–F granites of phase 3 are the most strongly enriched in rare metals and other incompatible elements and inherit geochemical tendencies toward enrichment or depletion in the same groups of elements from the parental biotite granites of the Kukulbey complex with respect to the Upper Crust (Fig. 3). They are enriched in Rb, Li, Cs, Be, Sn, W, Mo, Ta, Nb, Bi, and F but are depleted in Mg, Ca, Fe, Ti, P, Sr, Ba, V, Co, Ni, Cr, Zr, REE, and Y. The Zr/Hf ratio of rocks serves as a reliable indicator of the concentrating level of rare elements during the fractional crystallization of Kukulbey granites. Compared to the clarkes of trace elements in the Upper Crust, the Ta, Li, and Sn, concentrations in Li–F granites increase by factors of 75–35, the W, Rb, Be, and Pb concentrations are 20 to 12 times higher, and the F, Cs, Mo, Nb, and Hf concentrations are 10–2 times as high.
Fig. 1. Zr/Hf-SiO2 diagram for Shakhtama granites
Fig. 2. Zr/Hf-SiO2 diagra m for Kukulbey granites
Fig. 3. Spaidergram of Kukulbey granites
Fig. 4. Spaidergram of Shakhtama granites
Shakhtama basites and monzonites are impoverished in rare metal typical for granite of Kukulibey complex (Ta, Nb, Mo, Rb, Pb) but are enriched in elements peculiar to mantle rocks such as, Mg, Ti, Cr, V, Sr, Ba (Fig.4). Only in their more late differentiates - Q-syenites and Bt-Hbl granites is noted weak enrichment in Li, Mo, Ta, Rb, Ba, which, possible, is explained by greisenization connected with granites of Kukulbey complex.
Thereby, mismatch of the crystallization differentiation trends and of the geochemical particularities do not allow speak about genetic relationship of the Kukulibey granite with Shakhtama latite magmatism. In ditto time heat influence of the latite magmas could promote anatexsis melting of Kukulbey anhieutectic magma from silica-rich substratum of Upper Continental Crust (Zaraisky, 2004).
This study was financially supported by RFBR grant 08-05-00865 and Project 3763.2008.5 of the Program of Leading Research Schools.
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