Elsevier

Precambrian Research

Volume 243, April 2014, Pages 168-180
Precambrian Research

First report of Paleoproterozoic incipient charnockite from the North China Craton: Implications for ultrahigh-temperature metasomatism

https://doi.org/10.1016/j.precamres.2014.01.004Get rights and content

Highlights

Abstract

The assembly of crustal blocks within the North China Craton (NCC) at the terminal stage of craton building during Paleoproterozoic witnessed major subduction-collision events. Here we illustrate a case where hot and anhydrous magmas emplaced in the lower crust during the early Paleoproterozoic subduction realm in the NCC led to the formation of orthopyroxene-bearing dehydrated zones of incipient charnockites within the adjacent TTG (tonalite-trondhjemite-granodiorite) suite. The coarse greenish anhydrous zones of incipient charnockite show an assemblage of antiperthitic plagioclase + orthopyroxene + K-feldspar + quartz with minor clinopyroxene, ilmenite + magnetite and secondary biotite. The peak PT conditions of the incipient charnockite are estimated as 890–970 °C and 3.5–6.5 kbar based on mineral chemical analysis and pseudosection modeling in the system NCKFMASHTO, suggesting ultra-high temperature metasomatism, and the ‘hottest incipient charnockites’ so far reported. Zircons from the massive charnockite show oscillatory-zoned magmatic cores with high Th/U values rimmed by unzoned and luminescent thin metamorphic overgrowths. A concordant group of magmatic zircons yields weighted mean 207Pb/206Pb age of 2401 ± 29 Ma, and a lower intercept age of ca. 1834 ± 81 Ma. A similar concordant group of zircons from the TTG yields a weighted mean 207Pb/206Pb age of 2446 ± 18 Ma and a weighted mean lower intercept age of 1834 ± 47 Ma. The ca. 2.45 Ga age from the TTG and 2.40 age from the charnockite are taken to represent the magma emplacement ages, and the identical 1.83 Ga from zircons in both rocks denoting the timing of late Paleoproterozoic metamorphism. Lu–Hf analysis of magmatic cores of the zircons from the charnockite and the TTG show clear positive ɛHf(t) values (1.2–5.9), suggesting juvenile sources.

The formation of incipient charnockite zones close to the contact with the charnockite pluton is consistent with the quantitative numerical models that predict CO2 migration in anhydrous silicate melt and metasomatic fluid from a rapidly cooling pluton. The ultra-high temperature metasomatism and solid-state reaction generating the incipient charnockites provide important insights on CO2 advection from lower crustal magmas during Paleoproterozoic orogeny in the North China Craton.

Introduction

Charnockites are orthopyroxene-bearing anhydrous granitoids (Holland, 1900, Le Maitre, 2002) of magmatic or metamorphic origin (Frost and Frost, 2008). Charnockites belonging to the metamorphic group originate either from igneous or from sedimentary protoliths during high grade metamorphism under predominantly anhydrous conditions (Rajesh and Santosh, 2012, and references therein). Charnockitic rocks form common constituents of the exposed middle and lower crust in many Precambrian terranes and have been in focus for studies related to arc magmatism in convergent margins through time (e.g., Rajesh, 2012) as well as in evaluating lower crustal metamorphism and fluid rock interaction (e.g., Santosh and Omori, 2008, Touret and Huizenga, 2012). Charnockites range in age from Mesoarchean such as those in the Coorg Block of southern India, one of the oldest microcontinents on the globe (Santosh et al., 2013a) to late Cretaceous or even younger such as those in the Gangdese batholith of Tibet (Zhang et al., 2010). They form either massive plutons of large areal extent, or weakly to strongly foliated units in high grade metamorphic belts. Most of the Precambrian charnockites are characterized by dark olive-green to grayish-green or brownish-green color and greasy appearance with pristine to partly to nearly completely altered crystals of orthopyroxene (Rajesh and Santosh, 2012).

In a different scenario, Pichamuthu (1960) reported decimeter-sized veins, patches and ladders of charnockites developed within upper amphibolite facies gneisses in an orthogneiss quarry from Kabbaldurga in the Karnataka State of southern India leading to the introduction of new terminology in charnockite petrology, as ‘incipient charnockites’. The incipient charnockites show similar mineralogy and appearance to those of massive charnockites except that the greenish dehydration zones are clearly related to fluid-induced dehydration and recrystallization. In the subsequent years, such dehydration zones ranging in scale from a few centimeters to several meters were documented from several Precambrian granulite facies terranes over the world (Srikantappa et al., 1985, Santosh et al., 1990, Newton et al., 1980, Hansen et al., 1987, Stahle et al., 1987, Perchuk et al., 2000, Harlov et al., 2006, Tsunogae et al., 2012; among others; see Rajesh and Santosh, 2012 for a recent review). Petrological studies on these incipient charnockites have established metamorphic dehydration reaction involving the breakdown of hornblende and/or biotite with quartz and with or without garnet to produce orthopyroxene and/or clinopyroxene. In most cases, the dehydration is structurally controlled, mainly through buffering of water activity to low levels by the influx of CO2-rich fluids (Santosh et al., 1990, Newton, 1992, Santosh and Omori, 2008, Touret and Huizenga, 2012).

As one of the oldest cratonic nuclei on the globe, the North China Craton (NCC) preserves a prolonged history of Precambrian tectonothermal events and associated magmatic, metamorphic and metallogenic processes (Zhai and Santosh, 2011, Zhai and Santosh, 2013, Zhao and Zhai, 2013, Zheng et al., 2013). The Paleoproterozoic world in the North China Craton has been central to studies on the tectonics of assembly of Columbia – the Earth's earliest coherent supercontinent (Zhao et al., 2002, Rogers and Santosh, 2002, Rogers and Santosh, 2009, Nance et al., 2014, Roberts, 2013). Paleoproterozoic charnockites of both magmatic and metamorphic origin have been reported from the crustal blocks as well as in the intervening suture zones in the NCC, and have been related to a variety of tectonic processes associated with subduction-collision and post-collisional extension (e.g., Liu et al., 2011, Peng et al., 2012, Santosh, 2010, Santosh et al., 2013b, Zhao and Zhai, 2013). Here we report for the first time the finding of incipient charnockites from the NCC. We present petrological evidence which reveals that the incipient charnockites were generated under high to ultrahigh-temperature conditions from fast cooling charnockite magma. We present LA-ICPMS zircon U–Pb and Lu–Hf isotope data from the magmatic charnockite and TTG rocks which host the incipient charnockite which show that the emplacement of the pluton, and consequent dehydration of the surrounding rocks occurred during the early Paleoproterozoic, associated with the initiation of the closure between two major blocks in the NCC.

Section snippets

Geological setting

The NCC (Fig. 1) is a collage of three major crustal blocks – the Yinshan Block, the Ordos Block and the Eastern Block which were assembled into a coherent cratonic framework at the end of the Paleoproterozoic, broadly coinciding with the timing of amalgamation of the supercontinent Columbia (Zhao et al., 2002, Zhao et al., 2005, Zhao and Zhai, 2013, Zheng et al., 2013, Santosh, 2010, Santosh et al., 2012, Santosh et al., 2013a, Santosh et al., 2013b). The Yinshan Block and the Ordos Block were

Sampling and analytical techniques

Samples of the charnockite, TTG gneiss and incipient charnockite were collected for petrologic studies. One representative sample each from the massive charnockite (OY-XH-4a) and the TTG (OY-XH-5l) hosting the incipient charnockite were collected for zircon separation and LA-ICPMS U–Pb and Lu–Hf analysis.

Petrologic studies were carried out on polished thin sections. Chemical analyses of the minerals in the incipient charnockite were carried out using an electron microprobe analyzer (JEOL

Petrography

The incipient charnockite reported in this study from the NCC is a coarse-grained and holocrystalline rock composed mainly of plagioclase (50–60%), orthopyroxene (10–15%), clinopyroxene (5–10%), quartz (2–5%), K-feldspar (1–2%), and retrograde biotite (3–4%), with accessory apatite, Fe–Ti oxide (magnetite and ilmenite), and zircon (Fig. 5a). The rock probably corresponds to dioritic charnockite or enderbite because of the low modal abundance of K-feldspar and quartz. The mineral assemblage

Discussion

This is the first report of incipient charnockite formation associated with Paleoproterozoic tectonics in the North China Craton. The incipient charnockite patches and veins occur as coarse greenish domains carrying brownish orthopyroxene crystals within orthopyroxene-free TTG gneisses adjacent to the contact with an anhydrous charnockite pluton. The mineral assemblage in the incipient charnockite is orthopyroxene + antiperthitic plagioclase + K-feldspar + quartz, with minor clinopyroxene, ilmenite + 

Conclusions

  • 1.

    This is the first report of incipient charnockites from the North China Craton. We document centimeter- to decimeter-scale anhydrous zones of incipient charnockite within TTG rocks adjacent to an intrusive charnockite. The field structures and textural features are consistent with incipient charnockites reported from other Precambrian terranes.

  • 2.

    Petrologic studies and pseudosection analysis show that the incipient charnockites formed at ultrahigh-temperature conditions of 890–970 °C.

  • 3.

    Zircons from

Acknowledgements

We thank Editor Prof. Guochun Zhao and two referees from the Journal for their constructive comments. This study forms part of the PhD research work of Q.Y. Yang at the China University of Geosciences Beijing. This work also contributes to the Talent Award to M. Santosh under the 1000 Talents Plan of the Chinese Government. Partial funding for this project was produced by a Grant-in-Aid for Scientific Research (B) from Japan Society for the Promotion of Science (JSPS) to Tsunogae (No. 22403017).

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