Khondalite Belt Research Articles

Metamorphic P–T path and tectonic implications of pelitic granulites from the Daqingshan Complex of the Khondalite Belt, North China Craton

Pelitic granulites are cropped out in the Daqingshan Complex of the Khondalite Belt, a Paleoproterozoic tectonic belt in the Western Block of the North China Craton. Petrological studies show that these granulites contain four distinct metamorphic assemblages. The core of a garnet porphyroblast, along with fine-grained inclusions of biotite+plagioclase+K-feldspar+quartz±sillimanite±ilmenite, defines the prograde metamorphic (M1) stage. The peak (M2) assemblage consists of garnet (mantle)+biotite+K-feldspar+plagioclase+quartz±sillimanite±orthopyroxene±ilmenite±magnetite±rutile. Peak metamorphism was followed by a near-isothermal decompression (M3) and the development of coronae of garnet+biotite+cordierite+plagioclase+quartz±K-feldspar±sillimanite±spinel±ilmenite±magnetite (M3) in the Crd–Grt–Bt gneisses during the following garnet breakdown reactions: garnet+sillimanite+melt→cordierite+biotite+Fe-oxide and garnet+melt→biotite+quartz+plagioclase. Retrograde cooling (M4) assemblages are represented by biotite+muscovite+sillimanite+quartz+plagioclase+K-feldspar±ilmenite. Quantitative phase equilibria modeling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 was applied to obtain P–T conditions of <780°C and <9kbar for M1, 840–880°C and 9–11kbar for M2, 800–870°C and 5.0–7.5kbar for M3, and <660°C and 4.1–6.9kbar for M4. The combination of the mineral assemblages, mineral compositions, and metamorphic reaction histories in the Daqingshan pelitic granulites defines a clockwise P–T path that involves periods of near-isothermal decompression and late cooling that followed the peak granulite-facies metamorphism. This result is consistent with the tectonic history of the Daqingshan Complex in the Khondalite Belt, which involved continent–continent subduction and collision followed by exhumation and cooling. This suggests a continent–continent collisional event between the Yinshan and Ordos blocks, which became amalgamated to form the Paleoproterozoic Khondalite Belt in the Western Block of the North China Craton.

Geochronology and trace element geochemistry of zircon, monazite and garnet from the garnetite and/or associated other high-grade rocks: Implications for Palaeoproterozoic tectonothermal evolution of the Khondalite Belt, North China Craton

It is well documented that the Khondalite Belt of the North China Craton formed by the collision of the Yinshan and Ordos Blocks during ca. 1.95Ga, but the onset of the post-collisional exhumation stage has not been well dated. The garnetite and garnet-bearing quartz-rich lens that are regarded as residue-melt phases occur occasionally in the foliated metasedimentary rocks in the Jining terrane of the Khondalite Belt. They are free of regional gneissosity, and may be generated during the post-collisional exhumation stage. Zircon U–Pb dating, monazite chemical dating and trace element analyses of zircon, monazite and garnet have been carried out on the garnetites and associated other high-grade metamorphic rocks. The geochronological results reveal a major age group of ca. 1890Ma from the zircon grains/overgrowths and inclusion-type monazites within the garnet poikiloblasts in the garnetites. The trace elements analyses show that zircon hosted in the mantle of the garnet poikiloblasts in the garnetite contains the highest HREE and Y contents, whereas zircon hosted in the garnet rims shows higher HREE and Y contents than matrix zircon grains. Inclusion-type monazite in the garnetite shows higher LaN/SmN ratio than matrix monazite grains. Large garnet poikiloblasts in the garnetite exhibit bell-shaped HREE and Y zoning profiles attributed to Rayleigh fractionation during their growth. Compared to garnet poikiloblasts, coronal garnet shows higher Gro, Cr and Li contents, but lower XMg, Sm, HREE, Zn and Zr contents and weaker Eu negative anomalies, reflecting their different formation environments. Based on evidences from microstructural analyses, REE concentrations and patterns, HREE partition coefficients and the Ti-in-zircon thermometer, it is proposed that most of the zircon grains/overgrowths in the garnetite formed coevally with garnet poikiloblasts. Consequently, ca. 1890Ma represents the timing of garnet poikiloblast growth during the partial melting of the protolith. Coronal garnet or Zn-rich spinel plus cordierite coupled with the matrix-type monazite formed after garnet poikiloblasts growth during near-isothermal decompression. Given the high metamorphic temperature and pressure revealed by the garnetite (820–850°C and up to 950°C; 8.5–9.5kbar), it is suggested that the ca. 1890Ma age group represents the beginning of extension/exhumation event in the Khondalite Belt, which is also recorded in the associated other high-grade rocks such as the garnet-bearing quartz-rich lenses, sillimanite–garnet-bearing quartzo-feldspathic gneisses, garnet-bearing quartzo-feldspathic gneisses and a pure quartz vein.

Raman spectroscopic characterization of H2O in CO2-rich fluid inclusions in granulite facies metamorphic rocks

Water (H2O) has been identified during Raman analyses at ~150–200°C of fluid inclusions from granulites that were previously thought to contain pure CO2. At room temperature, H2O in a CO2-rich fluid inclusion forms an optically unresolvable liquid wetting the inclusion walls. During heating, the H2O phase evaporates into the CO2 phase to produce a homogeneous H2O–CO2 fluid throughout the inclusion volume, and the H2O Raman peak at ~3640cm−1 is clearly resolvable at elevated temperatures. To test and confirm the method, synthetic H2O–CO2 FI of known composition (87.5mol% and 95mol% CO2) were also analyzed.Previously studied granulite samples from the Adirondack Mountains, USA, and from the Kerala Khondalite Belt, Sittampundi Complex and Sevitturangampatti, India were tested for the presence of H2O in CO2-rich fluid inclusions. Traces of H2O were found in CO2-rich fluid inclusions in quartz crystals from the Adirondacks and from the Kerala Khondalite Belt. H2O was not detected in CO2-rich secondary fluid inclusions in garnet from the Sittampundi Complex and from Sevitturangampatti. Rather, hydrogarnet was detected along the walls of inclusions in garnet, as evidenced by a peak at ~3661cm−1. The hydrogarnet is interpreted to be a “step-daughter” phase that formed by reaction of the H2O in the fluid inclusion with the garnet host during retrogression. These observations confirm the presence of an H2O-bearing, CO2-rich fluid at some time during granulite petrogenesis, but do not resolve the question of whether these CO2-rich fluids were trapped at peak metamorphic conditions or during retrogression, or whether the fluid inclusions have reequilibrated following entrapment.

Earliest Paleoproterozoic supracrustal rocks in the North China Craton recognized from the Daqingshan area of the Khondalite Belt: Constraints on craton evolution

The Upper Wulashan “Subgroup” at Daqingshan in the Khondalite Belt of the Western Block of the North China Craton contains an Early Paleoproterozoic (2.5–2.45Ga) supracrustal sequence, incorporating banded iron formations, which we refer to as the Daqingshan Supracrustal Rocks. They contain rounded to elliptical zircon grains that commonly show core–rim or core–mantle–rim structures and precise SHRIMP U–Pb dating of these domains allows the discrimination of a series of tectono-thermal events that straddle the Archean/Proterozoic boundary. Detrital zircon cores with oscillatory zoning have formation ages ranging from 2.55 to 2.50Ga, indicating derivation from Late Neoarchean magmatic rocks. As a result of exhumation and erosion, these became incorporated in earliest Paleoproterozoic sediments that were deposited between 2.50 and 2.45Ga and then underwent high-grade metamorphism at 2.45–2.40Ga. This event variously recrystallized the cores into two main domains: a dark inner domain and a gray outer domain, as imaged in cathodoluminescence. The gray domains commonly show sector zoning and have Th/U ratios of 0.1–0.5, a feature commonly noted in zircons recrystallized under high-grade metamorphic conditions. Metamorphic mantles are generally more homogeneous in structure and have lower Th/U ratios (commonly <0.1) than the recrystallized domains, although they show similar age distributions. However, they are difficult to distinguish when only two domains are present in the zircon and they represent a continuum of recrystallization. The rocks then underwent a second episode of high-grade metamorphism in the Late Paleoproterozoic, as revealed by thin overgrowth rims that are homogeneous or show only weak zoning, have low Th/U ratios (commonly <0.1), and record ages of 1.95–1.90Ga. This is the first time that two tectono-thermal events of Early and Late Paleoproterozoic age have been identified in single rock samples from the Western Block of the North China Craton. The conclusion that unequivocal earliest Paleoproterozoic sediments are present in the North China Craton is also supported by evidence that the Daqingshan Supracrustal Rocks are cut by anatectic garnet granite, previously dated at 2.39Ga. Combined with evidence obtained here and elsewhere in the North China Craton, we conclude that the 2.45–2.40Ga metamorphism was not an extension of the well-established, craton-wide Late Neoarchean tectono-thermal event, but a previously unrecognized episode that followed a short quiescent period that allowed sedimentation of the Daqingshan Supracrustal Rocks.

Neoarchean–Paleoproterozoic multiple tectonothermal events in the western Alxa block, North China Craton and their geological implication: Evidence from zircon U–Pb ages and Hf isotopic composition

The Alxa block was traditionally considered to be part of the North China Craton, but its metamorphic basement has been poorly studied. Here we present a systematic zircon U–Pb and Hf isotopic investigation on four orthogneiss samples in the Beidashan area of the western Alxa block. The petrographic and geochemical data show that these rocks are granodioritic and trondhjemitic gneisses with TTG (tonalite–trondhjemite–granodiorite) characteristics. Zircons from the TTG gneisses display typical core–rim or core–mantle–rim structures. U–Pb datings and Hf isotopic analyses reveal two distinct age populations: the Latest Neoarchean (∼2.5Ga) and the Late Palaeoproterozoic (∼1.85Ga). The magmatic zircon cores and metamorphic mantles (rims) of the TTG gneisses were dated at similar ages around 2.5Ga, supporting the existence of Archean rocks in the western Alxa block. The short time interval between the Latest Neoarchean magmatism and the subsequent metamorphism suggests that they were related to the same Latest Neoarchean tectonothermal event. The ∼2.5Ga zircons have ɛHf(t) mainly between 0.8 and 5.0, TDM (Hf) model ages mainly between 2.6 and 2.8Ga (with a peak at ∼2.7Ga) and TDMC (Hf) model ages mainly between 2.7 and 3.0Ga (with a peak at ∼2.8Ga). The age of ∼1.85Ga obtained from two trondhjemitic gneisses is interpreted as the age of the Late Paleoproterzoic high-grade metamorphism. Our combined datasets show that the TTG gneisses in the Baidashan area of the western Alxa block experienced a main 2.7–2.8Ga crust growth, a ∼2.5Ga magmatic–metamorphic event and a ∼1.85Ga high-grade metamorphic event. The sequence of events is very similar to that of the other North China Craton. A Combination of the data of Paleoproterozoic metamorphic rocks in the Alxa block suggests that the Alxa block is the western extension of the Khondalite Belt rather than the Yinshan block.

New Constraints from Garnetite on the P-T Path of the Khondalite Belt: Implications for the Tectonic Evolution of the North China Craton

Garnetite pods containing >60 mol % garnet in association with garnet-bearing quartz-rich lenses are sporadically distributed in the Khondalite Belt, North China Craton. The mineral assemblages and microtextures of garnetite occurring in quartzo-feldspathic sillimanite–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, plagioclase, biotite, rutile and ilmenite defines an M1 assemblage consistent with partial melting and subsequent melt separation. Thermodynamic modeling of bell-shaped Ca zoning in garnet poikiloblasts confirms that this occurred during decompression. A retrograde M2 assemblage replacing M1 garnet or sillimanite is defined by two microdomains: coronas of Grt2 + Crd (M2a) where quartz is present, and Spl + Crd symplectite (M2b) where quartz is absent. The final metamorphic stage led to the coronal assemblage Grt3 + Opx + Crd + Pl + Bt (M3) surrounding M1 garnet. Comparison of the observed mineral proportions and compositions with the predictions of phase equilibria modeling in the NCKFMASHTO system for the effective bulk compositions, in combination with Zr-in-rutile thermometry, results in the following P–T conditions for the metamorphic events: 820–850°C (up to 950°C) and 8·5–9·5 kbar for M1; 850–865°C and 7·4–7·6 kbar for M2; 710–720°C and 6·4–6·6 kbar for M3. The clockwise P–T path thus defined for M1 to M3 evolution is interpreted to reflect the onset of extension and exhumation of the Khondalite Belt at ∼1890 Ma, following the emplacement of S-type granitoids at ∼1920–1890 Ma and ultrahigh-temperature metamorphism at ∼1920 Ma that resulted from collision of the Yinshan and Ordos Blocks at ∼1950 Ma.

Zircon U–Pb ages and Lu–Hf isotopes of Paleoproterozoic metasedimentary rocks in the Korla Complex, NW China: Implications for metamorphic zircon formation and geological evolution of the Tarim Craton

Widespread Paleoproterozoic supracrustal rocks in the northern Tarim Craton contain important information about its geological evolution and correlation with adjacent blocks. We present new in situ LA-(MC-)ICP-MS zircon U–Pb and Lu–Hf isotopic data for six mica schist samples from the Korla Complex. Field and petrological studies indicate a pelitic to semi-pelitic protolith and a high pressure upper amphibolite-facies peak metamorphic condition (T=690±50̊C and P=11±2kbar) for these samples. CL-images reveal that zircons in these samples are dominantly metamorphic origin and only a few detrital zircons occur as relics in sample T1, the ages of which suggest a maximum deposition age of ca. 2.0Ga and a sedimentary provenance from the Tarim Craton itself. All metamorphic zircons consistently record a metamorphic age of ca. 1.85Ga, despite of various degrees of discordance probably due to later Pb-loss. Both recrystallization and new zircon growth are recognized for the genesis of these metamorphic zircons. The metamorphic zircon domains in sample T1 show a relatively large range of initial 176Hf/177Hf ratios similar to the detrital cores, whereas those in the other samples show similar initial 176Hf/177Hf ratios (ca. 0.28140±0.00010, 2σ) regardless of their internal structures and degrees of discordance. The former is interpreted as a result of complete U–Pb resetting through fluid-mediated recrystallization, whereas the later probably implies a large-scale Hf isotopic homogenization during new zircon growth. Petrological and zircon isotopic evidence supports that the new zircon growth and Hf isotopic homogenization probably resulted from the mixing of Hf–Zr derived from dissolution of tiny detrital zircons and decomposition of garnet to chlorite in hydrothermal fluids during retrograde metamorphism. Accordingly, the ages of these new zircon growths may postdate the peak metamorphism, which was probably related to a late Paleoproterozoic collisional orogenic event in the northern Tarim Craton. A compilation of available geological and geochronological data enables us to identify two Late Paleoproterozoic orogenic belts: the ca. 1.9–1.8Ga North Tarim Orogen and the ca. 2.0–1.9Ga South Tarim Orogen. It is suggested that the Tarim Craton, including the Dunhuang and Quanji Blocks, was correlative with the Alxa–Yinshan Block of the North China Craton, and they probably formed a coherent massif during the Neoarchean–early Paleoproterozoic, which collided with the Ordos Block and its western extension along the ca. 1.95Ga Khondalite Belt–South Tarim Orogen to form a larger landmass in the Columbia Supercontinent.

Late Paleoproterozoic multiple metamorphic events in the Quanji Massif: Links with Tarim and North China Cratons and implications for assembly of the Columbia supercontinent

The Quanji Massif preserves important records of late Paleoproterozoic metamorphic events that provide insights into the tectonic history of the Tarim Craton and its relation with the North China Craton. Here we investigate amphibolites from two regions in the Quanji Massif. U–Pb ages from metamorphic zircons in the amphibolites, combined with previously published ages from this region, define distinct metamorphic events during Paleoproterozoic. The earlier medium-P/T type upper amphibolite-facies metamorphic event occurred at ca. 1.90–1.96Ga in a collisional setting. The second event took place with low-P/T type amphibolite-facies metamorphism in an arc setting at ca. 1.82–1.85Ga, with a subsequent medium-P/T type metamorphism during arc–continental collision at ca. 1.80–1.82Ga. Our data reveal for the first time that the late Paleoproterozoic metamorphic events in the Quanji Massif are closely comparable with those documented from the Khondalite Belt within the Inner Mongolia Suture Zone and the Trans-North China Orogen in the North China Craton. We therefore propose that the Quanji Massif and Tarim Craton probably shared similar tectonic histories with those of the North China Craton in the late Paleoproterozoic, coinciding with the global assembly of the Columbia supercontinent.

Contrasting styles of Phanerozoic and Precambrian continental collision

There are differences in the style of collisional orogens between the Phanerozoic and the Precambrian, most notably the appearance of blueschists and ultrahigh pressure metamorphic (UHPM) rocks in the geological record since the late Neoproterozoic, whereas these rocks are absent from older orogens. Understanding collisional orogenesis in the context of present-day values for ambient upper-mantle temperature and radiogenic heat production provides a reference from which to extrapolate back to conditions in the Precambrian. To evaluate differences in the way Phanerozoic and Precambrian collisional orogens develop, a series of experiments was run using a 2-D petrological–thermomechanical numerical model in which the collision of spontaneously moving continental plates was simulated for values of ambient upper-mantle temperature and radiogenic heat production increasing from those appropriate to the present-day. Thus, models of modern collisional orogens involving different modes of exhumation of UHPM rocks were extrapolated back to conditions appropriate for the Precambrian. Based on these experiments an increase of the ambient upper-mantle temperature to >80–100K above the present-day value leads to two distinct modes of collision that are different from the modern collision regime and for which the terms truncated hot collision regime (strong mafic lower continental crust) and two-sided hot collision regime (weak felsic lower continental crust) are proposed. Some Proterozoic orogens record post-extension thickening to generate counter-clockwise metamorphic P–T paths followed by slow close-to-isobaric retrograde cooling, such as occurred in the Paleoproterozoic Khondalite belt in the North China craton and the late Mesoproterozoic–early Neoproterozoic Eastern Ghats province, part of the Eastern Ghats belt of peninsular India. These orogens have similarities with the truncated hot collision regime in the numerical models, assuming subsequent shortening and thickening of the resulting hot lithosphere. Other Proterozoic orogens are characterized by clockwise looping metamorphic P–T paths and extensive granite magmatism derived from diverse crustal and subcontinental lithospheric mantle sources. These orogens have similarities with the two-sided hot collision regime in the numerical models. Both regimes are associated with shallow slab breakoff that precludes the formation of UHPM rocks. The temperature of the ambient upper-mantle where this transition in geodynamic regimes occurs corresponds broadly to the Neoproterozoic Era.

MnNCKFMASH Phase Relations in Cordierite–Orthopyroxene Migmatitic Gneisses, Southern India: Implications for Low-pressure Crustal Melting under Granulite-facies

Abstract Cordierite-orthopyroxene migmatitic gneisses exposed in Achankovil unit of the Kerala Khondalite Belt, southern India show evidences of melting, melt extraction and in-situ crystallization of melt under granulite-facies conditions. The sequential mineral assemblages garnet + biotite + orthopyroxene + plagioclase + quartz (± melt) in the mesosomes and garnet + biotite + orthopyroxene + cordierite + plagioclase + K-feldspar + quartz + melt in the melanosomes makes the Achankovil cordierite-orthopyroxene migmatitic gneisses a good example of anatectic rocks, where substantial melt fractions remained in-situ during decompression and cooling. Therefore, the rocks provide an opportunity to investigate deep crustal processes and record of rheological (thermal and mechanical) reequilibration prevailed during the final stages of orogeny. The significance of cordierite formation and its possible relationship with melt formation are investigated applying theoretical calculations in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnNCKFMASH) system. Results of numerical modelling of the mineral assemblages in pressure–temperature–composition (P–T–X) pseudosections using Perple_X infer that the sequence of reactions involving formation of cordierite-orthopyroxene-melt assemblage is consistent with an isothermal decompression (with a pressure drop of &amp;gt;1.5 kbars) at high temperatures (&amp;gt;800 °C), forming leucosomes. Biotite dehydration melting reactions, occurring above 4.5 kbars constrain prograde arm of the P–T trajectory and is interpreted as a product of crustal thickening, which was followed by rapid decompression. The final stage of exhumation is characterized by rehydration of cordierites in the melanosome by melt-solid interactions at exceptionally low-pressure (~3.2 kbars) conditions. The high-temperature isothermal decompression inferred from the mineral reactions and P–T–X pseudosections constitute a clockwise P–T path for the exhumation of the lower crust. This clockwise P–T path is consistent with the common tectonic model accepted for the genesis of granulite-facies migmatites during crustal thickening and later unroofing, accompanied with arc-continent collision. Our conclusions indicate low–P metamorphism and anatexis can be traced to convergent setting, where melt buoyancy considerably decreases density of the lithosphere and modifies rheology leading to rapid exhumation of the lower crust. Therefore, the crustal evolution in the Kerala Khondalite Belt is correlated with two stage processes: (i) thickening of the crust in relation to a continental-arc setting, followed by (ii) exhumation along a high-temperature stable geotherm with sufficient pressure release associated with syn- to post-convergence transpression and transtension.

Petrogenesis of high-K metagranites in the Kerala Khondalite Belt, southern India: a possible magmatic-arc link between India, Sri Lanka, and Madagascar

The Proterozoic Kerala Khondalite Belt (KKB), southern India preserves a distinct high-grade terrain that is interpreted to have been situated adjacent to Madagascar and Sri Lanka during Gondwana assembly. As such, it has become a major focus for testing models of supercontinent amalgamation and dispersal. The lithounits of KKB have remarkable petrological similarities to the Highland Complex (HC) of Sri Lanka and south-central Madagascar. However, there is no well-constrained petrogenetic model for the KKB that fits explicitly within a supercontinent reconstruction model. We present here results from our on-going studies on the origin and evolution of K-rich (potassic, where K2O/Na2O>1) gneisses of KKB in relation to Proterozoic supercontinent events. Our results show, in a major departure from earlier metasedimentary origin, that potassic gneisses are metamorphosed granitoids. The metagranitoid samples display high K2O contents and low Al2O3/(FeO+MgO+TiO2) values. They are moderate to strongly peraluminous (ASI values ranging from 1.05 to 1.47) rocks showing mineralogical, petrological, and geochemical characteristics distinctive of the high-K calc-alkaline suites. Typical of igneous suites, the high-K metagranites show minor variation in chemical compositions with most oxides showing negative correlation with SiO2. Geochemistry illustrates distinctive features of arc-related magmas with LILE (K, Rb, and Th) and LREE enriched patterns and considerable depletion of HSFE (Nb, Zr, and Ti). The high-K metagranites are further characterized by strong negative anomalies of Eu (Eu/Eu*=0.10–0.44) and Sr, suggesting melting in plagioclase stability field and retention of plagioclase in the residual phase. Petrogenetic discrimination for granitoids, using major and trace elements demonstrates that the high-K metagranites of the KKB formed by partial melting of igneous source in lower- to middle-crust levels. Overall the geochemical features are supportive of origin in relation to a convergent margin setting, possibly in a continental magmatic arc system, which can be connected to the amalgamation and dispersal of continental fragments in a supercontinent event. This study, therefore, provides a lead towards more robust comparisons between the Proterozoic supercontinent events and processes.

Amalgamation of the North China Craton: Key issues and discussion

Geological and geophysical data indicate that the Precambrian basement of the North China Craton (NCC) formed by amalgamation of a number of micro-continental blocks. The number of blocks, when they existed and how they came together are controversial, and in particular the following issues are disputed: (1) the timing of collisional event(s) leading to the amalgamation of the Eastern and Western blocks along the Trans-North China Orogen (TNCO); (2) the polarity of the subduction between the Eastern and Western blocks; (3) the validity of an old continental block (Fuping Block) that collided with the Eastern Block at ∼2.1Ga; (4) the tectonic setting of the northern margin of the NCC in the Paleoproterozoic; (5) the tectonic nature of high-pressure (HP) and ultrahigh temperature (UHT) granulite-facies events in the Khondalite Belt of the Western Block; and (6) the tectonic setting of the Paleoproterozoic Jiao-Liao-Ji Belt in the Eastern Block.Analysis and integration of available stratigraphic, structural, geochemical, metamorphic and geochronologic data enable the development of an internally consistent and coherent model for assembly and stabilization of the various Archean blocks of the NCC in the Paleoproterozoic. All metamorphic ages obtained for the TNCO are around 1.85Ga, which establishes that the final amalgamation of the Western and Eastern blocks of the craton occurred at ∼1.85Ga. The TNCO is characterized by a fan-shaped pattern of structural features, with the top-to-the-NW and top-to-the-SE thrusting in the northwest and southeast, respectively. This pattern does not constrain subduction polarity for the collisional assembly of the Eastern and Western blocks. Structures in lithospheric mantle and asthenosphere in the TNCO have been significantly modified/replaced in the Mesozoic and Cenozoic, and hence the present-day orientation of these structures, even if they relate to Paleoproterozoic assembly of the craton cannot be used to infer associated subduction polarity. There are no unique structural data or available metamorphic data to supporting the existence of an old continental block that intervened between the Eastern and Western Blocks, which collided with the Eastern Block at ∼2.1Ga. Available data are also inconsistent with the existence of the Paleoproterozoic Inner Mongolia-North Hebei Orogen along the northern margin of the NCC that formed through accretion of an exotic arc at ∼2.3Ga and incorporated into the Paleoproterozoic Columbia (Nuna) Supercontinent at 1.92–1.85Ga. We interpret the north Hebei portion of this inferred orogen as part of the TNCO, and the Inner Mongolian portion as an independent continental block (Yinshan Block). This block is separated from the Ordos Block by the Paleoproterozoic Khondalite Belt. The high-/medium-pressure granulite facies metamorphic event in the Khondalite Belt is considered to have resulted from collision between the Yinshan and Ordos blocks to form the Western Block at ∼1.95Ga, whereas the ∼1.92Ga UHT metamorphism within the belt was related to the underplating or intrusion of mantle-derived magmas during the post-collisional extension. The Jiao-Liao-Ji Belt in the Eastern Block likely formed through Paleoproterozoic rifting to form the Longgang and Langrim blocks, and subsequent basin closure and collision in the period 2.2–1.9Ga.

Episodic Paleoproterozoic (∼2.45, ∼1.95 and ∼1.85 Ga) mafic magmatism and associated high temperature metamorphism in the Daqingshan area, North China Craton: SHRIMP zircon U–Pb dating and whole-rock geochemistry

In the Daqingshan area within the Khondalite Belt of the North China Craton, Paleoproterozoic gabbro and dolerite intrusions show amphibolite to granulite-facies metamorphism. U–Pb zircon dating and whole-rock geochemistry were undertaken on these rocks in order to understand the timing of mantle magmatism and thermal processes. Ten samples yielded zircon U–Pb formation ages of 2.46–2.44Ga, 1.97–1.92Ga and 1.84Ga and metamorphic ages of 1.95–1.83Ga. They are variable in geochemical composition with some being enriched in light rare earth and large ion lithosphere elements but depleted in Nb–Ta. Combined with previous results, it is concluded that mafic magmatism at 2.45–2.37Ga and 1.97–1.92Ga indicates two Paleoproterozoic extensional phases, with the latter accompanied by HT–UHT metamorphism as a result of underplating of mantle magma.

Lithotectonic elements of Precambrian basement in the North China Craton: Review and tectonic implications

The North China Craton (NCC) consists of Archean to Paleoproterozoic basement overlain by Mesoproterozoic to Cenozoic cover. Minor Eoarchean to Mesoarchean basement rocks are locally present in the eastern part of the NCC, but little is known about their extent, nature and tectonic evolution due to widespread reworking by later events. The Neoarchean basement in the NCC was formed during two distinct periods: 2.8–2.7 Ga and 2.6–2.5 Ga, of which the former is considered as a major period of juvenile crustal growth in the NCC as evidenced by Nd and zircon Hf isotopic data, though the 2.8–2.7 Ga rocks are not widely exposed. The 2.6–2.5 Ga rocks make up ~80% of the Precambrian basement of the NCC and can be divided into high-grade gneiss complexes and low- to medium-grade granite-greenstone belts that are widespread over the whole NCC, seeming to support a notion that the cratonization of the NCC occurred at ~2.5 Ga. However, the 2.6–2.5 Ga rocks in the eastern and western parts of the NCC (Eastern and Western Blocks) are different from those similar-aged rocks in the central part (Trans-North China Orogen), with the former dominated by gneiss domes and metamorphosed at ~2.5 Ga, characterized by anticlockwise P–T paths involving isobaric cooling, reflecting an origin related to the underplating of mantle-derived magmas, whereas the latter, which are defined by strike-slip ductile shear zones, large-scale thrusting and folding, and transcurrent tectonics locally with sheath folds, were metamorphosed at ~1.85 Ga, characterized by clockwise P–T paths involving isothermal decompression, consistent with subduction and continent-continent collision settings. In addition, komatiites/komatiitic rocks are present in the granite-greenstone belts in the eastern and western parts of the NCC, but generally are absent in the central part. These differences imply that the 2.6–2.5 Ga basement rocks in the eastern and western parts of the NCC formed under different tectonic settings from those in the central part. Although both magmatic arc and mantle plume models can be used to explain the tectonic setting of the 2.6–2.5 Ga basement rocks in the eastern part of the NCC, a mantle plume model is favored as it can reasonably interpret: (1) the exceptionally large exposure of granitoid intrusions that formed over a short time period (2.55–2.50 Ga), without systematic age progression across a ~800 km wide block; (2) generation of komatiitic magmas with eruption temperatures as high as ~1650 °C; (3) dominant domal structures; (4) bimodal volcanic assemblages in the greenstone sequences; (5) affinities of mafic rocks to continental tholeiitic basalts; and (6) metamorphism with anticlockwise P–T paths involving isobaric cooling. In contrast, the 2.6–2.5 Ga high-grade gneiss terranes and low-grade granite-greenstone belts in the central part of the NCC exhibit the same structural and metamorphic characteristics as those of Paleoproterozoic lithological elements that typify active continental margin arcs and continent–continent collisional belts.Paleoproterozoic lithological assemblages in the NCC are mainly restricted to three Paleoproterozoic linear tectonic belts in the western, central and eastern parts of the NCC, which were, respectively, named the “Khondalite Belt (Fengzhen Belt/Inner Mongolia Suture Zone)”, “Trans-North China Orogen (Central Orogen Belt)” and “Jiao-Liao-Ji (Liaoji) Belt”. The three belts display some of the following lithotectonic elements that are classical indicators of subduction and collision tectonics in plate tectonic regimes: (1) arc-related juvenile crust; (2) linear structural belts defined by strike-slip ductile shear zones, large-scale thrusting and folding, and sheath folds and mineral lineations; (3) high-pressure (HP) mafic and pelitic granulites, retrograde eclogites and ultrahigh temperature (UHT) rocks; (4) clockwise metamorphic P–T paths involving near-isothermal decompression; (5) possible ancient oceanic fragments and mélange; and (6) back-arc or foreland basins. These lithotectonic elements indicate that subduction- and collision-related orogenic processes must have been involved in the development of the three Paleoproterozoic belts in the NCC. Different models have been proposed for the formation and evolution of these three Paleoproterozoic orogenic belts, and one of the models suggests that the Khondalite Belt was a continent–continent collisional belt along which the Yinshan and Ordos Blocks amalgamated to form the Western Block at ~1.95 Ga, which then collided with the exotic Eastern Block along the Trans-North China Orogen at ~1.85 Ga, whereas the Jiao-Liao-Ji Belt represents a rifting-and-collision belt within the Eastern Block which underwent rifting to form an incipient oceanic basin that was closed upon itself through subduction and collision at ~1.9 Ga. An alternative model proposes that all of the three Paleoproterozoic orogenic belts in the NCC were initialized from continental rifting on a single continent, which was cratonized through fusing Achaean microcontinental blocks at ~2.5 Ga, followed by the development of incipient oceanic basins which themselves were closed in the Paleoproterozoic through subduction and collision.

SHRIMP zircon U-Pb dating of late Paleoproterozoic kondalites in the Daqing Mountains area on the North China Craton

The Daqing Mountains area comprises a typical occurrence of the Khondalite Belt in the Western Block of the North China Craton (NCC). In this area, both early and late Paleoproterozoic metasedimentary rocks have been identified in what was originally called the Upper Wula Mountains “Subgroup”. Six metasedimentary rock samples yielded SHRIMP U-Pb zircon ages of 2.56–2.04 Ga for detrital and 1.96–1.83 Ga for metamorphic zircons. Based on these data and previously published results, the following conclusions can be drawn: 1) The source region for the late Paleoproterozoic detrital sedimentary rocks is mainly 2.55–2.4 and 2.2–2.04 Ga in age, consistent with the early Precambrian geological history identified widely in the basement of the NCC. 2) The majority of sedimentary rocks of the khondalite series were deposited between 2.04 and 1.95 Ga, and then in a protracted period (1.96 and 1.83 Ga) underwent a complex history of amphibolite to granulite-facies metamorphism.

Paleoproterozoic granulites from Heling'er: Implications for regional ultrahigh-temperature metamorphism in the North China Craton

The Inner Mongolia Suture Zone (IMSZ, incorporating the Khondalite Belt) forms part of an accretionary orogen developed during the Paleoproterozoic collision between the Yinshan Block and Ordos Blocks within the North China Craton (NCC). The granulites within the IMSZ are characterized by high-temperature (HT) to ultrahigh-temperature (UHT) metamorphism with diagnostic mineral assemblages for UHT conditions preserved in some localities such as Tuguiwula and Wuchuan. We report here a new locality in Heling'er, located within the southern domain of the IMSZ where spinel–garnet–sillimanite-bearing granulites record UHT metamorphic conditions. The prograde assemblages occurring as inclusions in porphyroblastic minerals in these rocks include sillimanite+quartz+spinel, whereas coarse grained garnet+sillimanite+quartz+spinel+cordierite+plagioclase+K-feldspar+magnetite+ilmenite represent the peak assemblages. The retrograde stage is marked by orthopyroxene and cordierite rims developed on garnet and spinel. Biotite marks the final hydration stage of the high-grade metamorphism. Spinel and quartz occur in direct contact, with the spinel showing low Zn, Cr, and Fe3+. Application of Ti-in-zircon and ternary feldspar geothermometers yields temperatures up to ~940°C for the peak stage. Mineral phase equilibria modeling based on the bulk chemistry and mineral assemblages shows that the peak assemblage of garnet+plagioclase+spinel+cordierite+sillimanite+quartz+ilmenite+magnetite was stabilized at 1030°C at 6.5–7.5kbar, thus we infer a 940–1030°C temperature range for peak metamorphism in these rocks. The newly discovered UHT rocks in Heling'er, dated as 1.92Ga, together with the two previously reported UHT localities at Tuguiwula and Wuchuan, suggest that the Paleoproterozoic UHT metamorphism occurred on a regional scale within the IMSZ, probably in association with the subduction–accretion–collision tectonics and the incorporation of the NCC within the Columbia supercontinent.

The crustal accretion history and tectonic evolution of the NE China segment of the Central Asian Orogenic Belt

The basement rocks in parts of NE China constitute a khondalitic sequence of sillimanite- and garnet-bearing gneisses, hornblende–plagioclase gneiss and various felsic paragneisses. Zircon U–Pb dating of garnet–sillimanite gneiss samples from the Erguna, Xing'an, Jiamusi and Khanka blocks indicates that high-grade metamorphism occurred at ~500Ma. Evidence from detrital zircons in Paleozoic sediments from the Songliao Block also indicates the former presence of a ~500Ma component. This uniformity of U–Pb ages across all crustal blocks in NE China establishes a >1300km long Late Pan-African khondalite belt which we have named the ‘NE China Khondalite Belt’. This indicates the blocks of NE China were amalgamated prior to ~500Ma, contrary to current belief. One scenario is that this amalgamated terrane had a tectonic affinity to the Siberia Craton, once forming part of the Late Pan-African (~500Ma) Sayang–Baikal orogenic belt extensively developed around the southern margin of the Siberia Craton. This belt was the result of collision between currently unidentified terranes with the Southeastern Angara–Anabar Province at about 500Ma, where the rocks were deformed and metamorphosed to granulite facies. It appears likely that at sometime after ~450Ma, the combined NE China blocks rifted away from Siberia and moved southward to form what is now NE China. The combined block collided with the North China Craton along the Solonker–Xar Moron–Changchun suture zone at ~230Ma rather than in the end-Permian as previously thought. Local rifting at the eastern extremity of the developing Central Asian Orogenic Belt (CAOB) resulted in the splitting away of the Jiamusi/Khanka(/Bureya) blocks. However, this was only transient and sometime between 210 and 180Ma, these were re-united with the CAOB by the onset of Pacific plate subduction, which has dominated the tectonic evolution of the region since that time.

Decoding multiple tectonothermal events in zircons from single rock samples: SHRIMP zircon U–Pb data from the late Neoarchean rocks of Daqingshan, North China Craton

A complex sequence of tectonothermal events shaped the Neoarchean–Paleoproterozoic crustal evolution in the North China Craton (NCC). Here we present a systematic SHRIMP zircon U–Pb investigation on a suite of metamorphic rocks from the Hademengou area of Daqingshan within the Khondalite Belt of the Inner Mongolia Suture Zone. We analyzed zircons from two meta-volcanic rocks of the Sanggan “Group” and four meta-igneous rocks of late Neoarchean to early Paleoproterozoic. Zircons from these rocks display typical core–rim or core–mantle–rim structures. The relict magmatic zircons record crystallization ages of ca. 2.5Ga followed by variable Pb loss, showing a range of ages in the concordia up to 2.1Ga. The recrystallized domains of zircons show sector zoning or are composed of inner dark domains, and outer lighter domains. The overgrowth mantles are homogeneous. The recrystallized zircons and overgrowth mantles show similar age data distribution along a concordia from ~2.5 to ~2.1Ga. The overgrowth rims are poorly developed but record ages (1.96–1.84Ga) similar to those displayed by metamorphic zircons in the late Paleoproterozoic “Upper Wulashan Subgroup” as well as the charnockites and UHT granulites reported in previous studies from this region. A meta-syenogranite intruding the Sanggan “Group” and Hypersthene dioritic gneiss contains 1.95Ga magmatic and 1.85Ga metamorphic zircons. Magmatic zircons of the late Neoarchean–early Paleoproterozoic metavolcanic rocks have εHf(t) of 1.2 to 7.0, tDM1(Hf) model ages of 2687 to 2532Ma and tDM2(CC)(Hf) model ages of 2837 to 2559Ma. The recrystallized zircons and mantle domains display Hf isotope composition similar to that of the magmatic domains. Our study is the first report from this region that identifies multiple tectonothermal events ranging from late Neoarchean through early to late Paleoproterozoic from zircons within single rock samples in the NCC. A strong overprint of late Paleoproterozoic metamorphism traced commonly in all the rock units indicates that these rocks were involved in the tectonothermal event associated with the final amalgamation of the NCC.

Spinel + quartz-bearing ultrahigh-temperature granulites from Xumayao, Inner Mongolia Suture Zone, North China Craton: Petrology, phase equilibria and counterclockwise p-T path

The Khondalite Belt within the Inner Mongolia Suture Zone (IMSZ) in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature (UHT) metamorphism. Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ. Petrological studies and thermodynamic modeling of the spinel + quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions. Spinel occurs in two textural settings: 1) high XZn(Zn/(Mg + FeII + Zn) = 0.071–0.232) spinel with perthitic K-feldspar, sillimanite and quartz in the rock matrix; and 2) low XZn (0.045–0.070) spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite.Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks: 1) near-isobaric cooling from 975 °C to 875 °C around 8 kbar, represented by the formation of garnet porphyroblasts from spinel and quartz; and 2) cooling and decompression from 850 °C, 8 kbar to below 750 °C, 6.5 kbar, represented by the break-down of garnet. The spinel + quartz assemblage is considered to have been stable at peak metamorphism, formed through the break-down of cordierite, indicating a near isothermal compression process. Our study confirms the regional extent of UHT metamorphism within the IMSZ associated with the Paleoproterozoic subduction-collision process.

Microstructures of melt inclusions in anatectic metasedimentary rocks

AbstractThe occurrence of crystallized and glassy melt inclusions (MI) in high‐grade, partially melted metapelites and metagraywackes has opened up new possibilities to investigate anatectic processes. The present study focuses on three case studies: khondalites from the Kerala Khondalite Belt (India), the Ronda migmatites (Spain), and the Barun Gneiss (Nepal Himalaya). The results of a detailed microstructural investigation are reported, along with some new microchemical data on the bulk composition of MI. These inclusions were trapped within peritectic garnet and ilmenite during crystal growth and are therefore primary inclusions. They are generally isometric and very small in size, mostly ≤15 μm, and only rarely reaching 30 μm; they occur in clusters. In most cases inclusions are crystallized (‘nanogranites’) and contain a granitic phase assemblage with quartz, feldspar and one or two mica depending on the particular case study, commonly with accessory phases (mainly zircon, apatite, rutile). In many cases the polycrystalline aggregates that make up the nanogranites show igneous microstructures, e.g. granophyric intergrowths, micrographic quartz in K‐feldspar and cuneiform rods of quartz in plagioclase. Further evidence for the former presence of melt within the investigated inclusions consists of melt pseudomorphs, similar to those recognized at larger scale in the host migmatites. Moreover, partially crystallized inclusions are locally abundant and together with very small (≤8 μm) glassy inclusions may occur in the same clusters. Both crystallized and partially crystallized inclusions often display a diffuse nanoporosity, which may contain fluids, depending on the case study. After entrapment, inclusions underwent limited microstructural modifications, such as shape maturation, local necking down processes, and decrepitation (mainly in the Barun Gneiss), which did not influence their bulk composition. Re‐homogenized nanogranites and glassy inclusions show a leucogranitic and peraluminous composition, consistent with the results of partial melting experiments on metapelites and metagraywackes. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes.