New Constraints from Garnetite on the P^T Path of the Khondalite Belt: Implications for the Tectonic Evolution of the North China Craton
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JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 PAGES1725^1758 2013 doi:10.1093/petrology/egt029 New Constraints from Garnetite on the P^T Path of the Khondalite Belt: Implications for the Tectonic Evolution of the North China Craton 1 1 2 SHUJUAN JIAO *, JINGHUI GUO ,SIMONL.HARLEY AND Downloaded from BRIAN F.WINDLEY3 1STATE KEY LABORATORY OF LITHOSPHERIC EVOLUTION, INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, BEIJING, 100029, CHINA 2GRANT INSTITUTE OF EARTH SCIENCE, SCHOOL OF GEOSCIENCES, UNIVERSITY OF EDINBURGH, http://petrology.oxfordjournals.org/ KINGS BUILDINGS, WEST MAINS ROAD, EDINBURGH EH9 3JW, UK 3DEPARTMENT OF GEOLOGY, UNIVERSITY OF LEICESTER, LEICESTER LE1 7RH, UK RECEIVED MARCH 5, 2012; ACCEPTED APRIL 12, 2013 ADVANCE ACCESS PUBLICATION MAY 21, 2013 Garnetite pods containing 460 mol % garnet in association with at 1890 Ma, following the emplacement of S-type granitoids at Institute of Geology and Geophysics, CAS on August 27, 2013 garnet-bearing quartz-rich lenses are sporadically distributed in the at 1920^1890 Ma and ultrahigh-temperature metamorphism at Khondalite Belt, North China Craton. The mineral assemblages 1920 Ma that resulted from collision of the Yinshan and Ordos and microtextures of garnetite occurring in quartzo-feldspathic silli- Blocks at 1950 Ma. manite^garnet gneiss in the Xiaoshizi area allow definition of the metamorphic history of the area. Formation of garnet poikiloblasts coupled with their enclosing and matrix minerals quartz, sillimanite, KEY WORDS: clockwise P^T path; garnetite; Khondalite Belt; North plagioclase, biotite, rutile and ilmenite defines an M1 assemblage China Craton; pseudosection consistent with partial melting and subsequent melt separation. Thermodynamic modeling of bell-shaped Ca zoning in garnet poiki- loblasts confirms that this occurred during decompression. A retro- INTRODUCTION grade M2 assemblage replacing M1 garnet or sillimanite is defined The Khondalite Belt (Fig. 1a) of the North China Craton by two microdomains: coronas of Grt2 þ Crd (M2a) where quartz has been studied intensively in recent years, especially fol- is present, and Spl þ Crd symplectite (M2b) where quartz is lowing the discovery of ultrahigh-temperature (UHT) absent. The final metamorphic stage led to the coronal assemblage metamorphic rocks (Santosh et al., 2007a, 2009a;Guo Grt3 þ Opx þ Crd þ Pl þ Bt (M3)surroundingM1 garnet. et al., 2012).The North China Craton (Fig. 1a) was divided Comparison of the observed mineral proportions and compositions into the Eastern and Western Blocks, separated by the with the predictions of phase equilibria modeling in the Trans-North China Orogen, by Zhao et al.(2001).Zhao NCKFMASHTO system for the effective bulk compositions, in et al. (2005) further divided the Western Block into the combination with Zr-in-rutile thermometry, results in the following Yinshan and Ordos Blocks, separated by the Khondalite P^T conditions for the metamorphic events: 820^8508C (up to Belt, and inferred that the Khondalite Belt was a colli- 9508C) and 8·5^9·5 kbar for M1;850^8658Cand7·4^7·6kbar sional orogen, like the Trans-North China Orogen, based for M2;710^7208Cand6·4^6·6 kbar for M3.The clockwise P^T on the metamorphic P^T paths, lithologies, structure and path thus defined for M1 to M3 evolution is interpreted to reflect geochronology of the high-grade rocks in the belt, and pro- the onset of extension and exhumation of the Khondalite Belt posed that it formed before the amalgamation of the ß The Author 2013. Published by Oxford University Press. All *Corresponding author. Telephone: þ86 -(0)10 - 82998521. Fax: rights reserved. For Permissions, please e-mail: journals.permissions@ þ 86-(0)10-62010846. E-mail: [email protected] oup.com JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013 Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013 Fig. 1. (a) Geological and tectonic map of the North China Craton (after Zhao et al., 2005). (b) Tectonic map of China showing the major cra- tons and orogens (after Zhao et al., 2001). CAOB, Central Asian Orogenic Belt; HO, Himalaya Orogen; NCC, North China Craton; SCC, South China Craton; SLO, Su^Lu Orogen; TC, Tarim Craton; QDO, Qinling^Dabie Orogen; QO, Qilianshan Orogen. Metamorphic com- plexes: AL, Alashan; DU, Daqingshan^Ulashan; GY, Guyang; HL, Helanshan; JN, Jining; QL, Qianlishan; WC, Wuchuan. North China Craton. In their discussion of the presence of Khondalite Belt, especially its metamorphic P^T^t trajec- anti-clockwise P^T paths, Zhao et al. (1998, 2001,2005) sug- tories, to accurately constrain the tectonic evolution. gested that the metamorphism of the Western (mainly the Furthermore, the peak temperatures calculated from the Yinshan Block, as the Ordos Block is covered by the metamorphic rocks in theJining terrane^eastern part of the Mesozoic to Cenozoic strata of the Ordos Basin) and Khondalite Belt (Figs 1 and 2) have not been well con- Eastern Blocks (Fig. 1a) was related to the intrusion and strained, as they have been largely based on conventional underplating of mantle-derived magmas, whereas the Fe^Mg exchange thermobarometers. The low closure tem- clockwise P^T paths involving near-isothermal decom- peratures of these geothermobarometers (less than 8008C) pression from the Khondalite Belt and the Trans-North overprinted by subsequent cation diffusion (Lu et al., 1992, China Orogen (Fig. 1a) reflected continental collision tec- 1996; Lu & Jin,1993) have consequences for the inferred P^ tonics. Santosh (2010) interpreted the Inner Mongolia T paths not only because of the uncertainties in estimated Suture Zone (i.e. the Khondalite Belt) as a subduction^ temperature, but also because of the impacts on pressure cal- accretion^collision orogen between the Yinshan and culations that result from Fe^Mg feedback effects (Frost & Ordos Blocks. In his model the tonalite^trondhjemite^ Chacko, 1989; Harley, 1989; Fitzsimons & Harley, 1994). granodiorite (TTG) gneiss, charnockite and calc-alkaline Advances in thermodynamic analysis, for example, granite in the Suture Zone (i.e. the Khondalite Belt) repre- THERMOCALC 3.33 (Powell & Holland,1988) and in par- sented an accreted oceanic plate sequence including a con- ticular the updated internally consistent thermodynamic tinental arc built up through subduction from the north. dataset (e.g. Holland & Powell, 1998), provide powerful Santosh (2010) also speculated on the basis of recent seis- quantitative methods to calculate P^T paths with greater mic data that the North China Craton developed by confidence and precision. THERMOCALC makes it pos- double-sided subduction between major crustal blocks. sible to correlate the bulk-rock compositions with their most The existence of these controversial models for its genesis stable mineral assemblages over a range of P^T space (i.e. and geological context necessitates a further study of the P^T pseudosection calculation), and to use the contouring 1726 JIAO et al. KHONDALITE BELT GARNETITE Downloaded from http://petrology.oxfordjournals.org/ Fig. 2. Distribution of the high-grade metamorphic rocks in the eastern segment of the Khondalite Belt (Jining terrane), North China Craton (modified after Guo et al., 2001a). at Institute of Geology and Geophysics, CAS on August 27, 2013 of calculated phase diagrams for cations that have low diffu- Mineral abbreviations are based on those of Whitney & sivities, such asVIAl in orthopyroxene, and Ca in plagioclase Evans (2010). andgarnet, toprovide additionalconstraints onthe P^Tcon- The second garnetite type occurs along with garnet- ditions. In addition, the pseudosection approach not only bearing quartz-rich lenses as irregular pods within gives the peak metamorphic conditions, but also P^T quartzo-feldspathic sillimanite^garnet gneiss (e.g. Xiao- path information, as well as allowing investigation of shizi area; Figs 2 and 3). This garnetite type has a processes such as partial melting and melt loss (Powell & distinctive mineral assemblage consisting of Grt þ Sil þ Holland,2008). Pl þ Qz þ Bt þ Ilm, with minor rutile, spinel, cordierite Garnetite with garnet contents up to 60 mol % is very and orthopyroxene. The nearby garnet-bearing quartz- rare and sporadically distributed in the metasedimentary rich lenses contain abundant quartz and minor gar- rocks of the Khondalite Belt. In detail, two types of garne- net associated with rare biotite, K-feldspar, rutile and tite are identified. The first garnetite type occurs as hori- ilmenite. zons or lenses between gabbro dykes and their host This study focuses on the second garnetite type, from the metapelites (e.g. Tuguishan; Fig. 2), and contains abundant Xiaoshizi area (Jining terrane of the Khondalite Belt; garnet, as well as orthopyroxene, rutile and plagioclase. Figs 1^3), and utilizes its mineral assemblages and micro- Rivalenti et al. (1997) investigated similar garnetites in the textures to deduce the P^Tevolution. The detailed petrog- Ivrea^Verbano Zone (Val Fiorina), NW Alps. The mineral raphy, reaction analyses coupling mineral chemistry with assemblage in that case was Grt þ Bt þ Pl Æ Cpx Æ Hb Æ pseudosection calculations and independent geother- Opx, and Rivalenti et al. (1997) suggested that these mometric estimates (i.e. Zr-in-rutile) are presented, and a garnetites may have formed by two processes: (1) meta- P^T path is derived using a combination of all of these somatism through interstitial melt retention in amphibo- constraints. The metamorphic evolution and petrogenesis lites and metapelites undergoing synchronous anatexis; (2) of the garnetites, and the implications for the tectonic evo- reaction between metapelite-derived anatectic melt and lution of the Khondalite Belt between the Yinshan and phases segregated from a basaltic sill during its intrusion, Ordos Blocks, are discussed in the light of this new which induced partial melting of the country rocks. evidence. 1727 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013 Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013 Fig.