Monzogranitic Gneisses Research Articles

Late Neoarchean crust-mantle geodynamics: Evidence from Pingquan Complex of the Northern Hebei Province, North China Craton

A late Neoarchean intra-oceanic arc along the northwestern margin of Eastern Block (EB), North China Craton, provides important insights into the nature of Archean mantle sources and crust-mantle geodynamics. The Pingquan Complex and the entire Northern Hebei Province (NHB) are located in the middle part of the arc, and overlap the northern extent of the trans-North China Orogen. Zircon U-Pb isotopic age data reveal that the Pingquan Complex consists of ∼2537–2515Ma dioritic gneisses, ∼2506–2503Ma amphibolites, and ∼2491Ma quartz monzodioritic to monzogranitic gneisses, and they show dominantly positive zircon εHf(t) (−0.6 to +5.4) that are lower than coeval model depleted mantle values.Geochemical data for the Pingquan rocks and synchronous metabasalts and granitoid gneisses of Huai’an-Xuanhua and Dantazi complexes in the NHB are integrated. Except for the monzogranitic gneisses that were derived from partial melting of juvenile metagreywackes, the other rocks of the Pingquan Complex were derived from a metasomatized lithospheric mantle, and subjected to variable fractionation of clinopyroxene, hornblende and plagioclase, without significant crustal contamination. Moderately depleted zircon εHf(t), and high Sm/Hf and Nb/Ta (mostly of 1.34–3.96 and 15.50–32.58, respectively) suggest that the lithospheric mantle was enriched by subducted pelagic sediments metamorphosed to rutile-bearing eclogites before melting.Late Neoarchean crust-mantle geodynamic processes in the NHB are reconstructed. Intra-oceanic subduction initiated offshore the northwestern margin of the EB at ∼2.55Ga or earlier. Partial melting of slab basalts occurred at ∼2542–2499Ma, with the melts contaminated by mantle wedge materials forming TTGs. Meanwhile, the sub-arc lithospheric mantle was enriched by fluids and melts released from slab basalts and pelagic sediments, and partial melting of this moderately depleted mantle generated ∼2537–2503Ma diorites and basalts. Following final accretion of the arc onto the continental margin of the EB, the slab rollback/breakoff and asthenospheric mantle upwelling triggered partial melting of the metasomatized lithospheric mantle and crustal anatexis, generating ∼2491Ma quartz monzodioritic and monzogranitic rocks.Accordingly, the NHB records Neoarchean crustal growth linked to oceanic subduction and arc-continent accretion, and highlights the importance of resolving the nature of mantle sources and crust-mantle interactions in understanding Archean crustal growth and evolution.

Palaeoproterozoic granitic magmatism in the northern segment of the Jiao-Liao-Ji Belt: implications for orogenesis along the Eastern Block of the North China Craton

ABSTRACTAs the northern segment of the Jiao-Liao-Ji Belt (JLJB), the Palaeoproterozoic Liaoji Belt is a key region for deciphering the formation and evolution of the North China Craton (NCC). In this study, we present the geochronology, geochemical, and isotopic studies on the monzogranitic gneiss, which is one of the major lithotectonic elements of the Liaoji Belt. LA-ICP-MS zircon U–Pb dating reveals that the studied monzogranitic gneisses were formed in the period of 2213–2178 Ma. They are in tectonic contact with the Palaeoproterozoic volcano-sedimentary rocks in the field. The monzogranitic gneisses belong to the high-K calc-alkaline series, and are metaluminous to peraluminous. They have 10,000 Ga/Al ratios of 2.63–3.14 with an average of 2.90, and are thus classified as aluminous A-type granites. Their εNd(t) values vary from −3.4 to +2.5, indicating heterogeneous source region. The monzogranitic gneisses are characterized by enrichment in LREE and LILE (e.g. Rb, Ba, Th, and K) and depletion in HREE and HFSE (such as Nb, Ta, and Ti), and are typical to magmatism in active continental margins formed in a subduction-related tectonic setting. Taking into account their A-type affinity and regional geological data, we suggest that the monzogranitic gneisses were most probably generated in a local extensional back-arc environment during subduction.

Late Neoarchean monzogranitic–syenogranitic gneisses in the Eastern Hebei–Western Liaoning Province, North China Craton: Petrogenesis and implications for tectonic setting

The medium-grained monzogranitic–syenogranitic gneisses expose as roughly northeast–southwest-trending and cover ∼1500km2 in the southern part of the Eastern Hebei–Western Liaoning Province, North China Craton. These plutonic gneisses have leucocratic features and weakly gneissic to massive structures, and were metamorphosed under greenschist to lower amphibolite facies. LA-ICP-MS zircon U-Pb isotopic dating reveals that the magmatic precursors of these monzogranitic–syenogranitic gneisses were emplaced during 2527–2511Ma. They are characterized by high contents of SiO2 (71.64–78.08%) and K2O (3.55–6.40%), and low contents of CaO (0.25–1.71%), MgO (0.01–0.42%), and Na2O (2.29–4.96%). According to their chondrite-normalized REE patterns, the monzogranitic–syenogranitic gneisses can be subdivided into a high REE group and a low REE group. Studies of petrogenesis reveal that magmatic precursors of the high REE group samples were derived from partial melting of the regionally widespread porphyritic or medium-grained dioritic–quartz monzonitic–granodioritic–monzogranitic gneisses under low pressure conditions (less than ca. 10kbar). The magmatic precursors of the low REE group were formed by fractionated crystallization of the high REE group magma, with titanite, allanite, epidote, apatite, and zircon as the major crystallization fractionation phases. Combined with previous investigations on the late Neoarchean metavolcanic rocks and granitoid orthogneisses in the northwestern part of the Eastern Hebei–Western Liaoning Province, the magmatic precursors of these monzogranitic–syenogranitic gneisses were most likely formed in a back-arc basin along the north of the Eastern Block, North China Craton.

Late Neoarchean subduction-related crustal growth in the Northern Liaoning region of the North China Craton: Evidence from ∼2.55 to 2.50 Ga granitoid gneisses

The North China Craton (NCC), dominated by ∼2.6–2.5Ga tectonothermal events, provides a natural laboratory to study Neoarchean crustal growth and geodynamic evolution. Late Neoarchean granitoid gneisses are well exposed in the Northern Liaoning Province, located north of the ancient Anshan–Benxi terrane along the northeastern margin of the Eastern Block (EB) of the NCC. LA-ICPMS zircon U–Pb isotopic data reveal that granitoid gneisses in the Qingyuan area can be grouped into two major episodes, i.e., ∼2559–2534Ma strongly gneissic quartz dioritic and tonalitic to trondhjemitic gneisses; and ∼2529–2495Ma weakly gneissic to massive quartz monzodioritic and monzogranitic gneisses, with subordinate tonalitic to trondhjemitic gneisses. The late magmatic episode was accompanied by regionally high-grade metamorphism (∼2510–2495Ma). Most granitoid gneisses display highly depleted zircon εHf(t2) values (+4.2 to +8.1), whereas one monzogranitic gneiss shows negative values of −4.7 to −1.0, indicating late Neoarchean crustal growth with minor involvement of ancient continental materials probably sourced from the Anshan–Benxi terrane.Geochemical and petrogenetic studies reveal that the quartz dioritic magmas were derived from partial melting of plagioclase-poor garnet amphibolites or eclogites metamorphosed from oceanic slab materials, with slab melts contaminated by mantle wedge peridotites during ascent. The tonalitic to trondhjemitic magmas stemmed from partial melting of mainly juvenile metabasaltic rocks with minor metagreywackes of lower arc crust. In comparison, the quartz monzodioritic and monzogranitic magmas were derived respectively from partial melting of depleted mantle sources metasomatized by slab-derived fluids and metagreywackes with different crustal resident ages at middle to lower crustal levels.Combined with previous studies of supracrustal metavolcanic rocks, the Northern Liaoning Province records late Neoarchean crustal growth, evolving from mid-ocean ridge, through initiation and maturation of an intra-oceanic arc, to arc–continent collision. Arc–continent accretion and possibly slab rollback processes may have triggered reworking of both juvenile arc crust and minor ancient continental margin materials, generating the magmatic precursors for the monzogranitic gneisses. Overall, the intense late Neoarchean crustal growth of the EB was controlled mainly by arc–continent accretion, possibly linked to global assembly of cratonic fragments.

Geochronology, petrogenesis and tectonic implications of Paleoproterozoic granitoid rocks in the Jiaobei Terrane, North China Craton

Paleoproterozoic granitoid rocks in the Jiaobei Terrane of the North China Craton mainly comprise deformed hornblende/biotite-bearing monzogranitic gneisses, undeformed biotite-bearing syenogranite, and tourmaline-bearing pegmatitic granite. In order to define the emplacement ages, sources, and petrogenesis of the granitoids, we have conducted a coupled LA-ICP-MS U–Pb dating, trace element and in situ Hf isotopic study of zircons from three representative Paleoproterozoic granitoid rocks in the Jiaobei Terrane. Our new zircon U–Pb data yield emplacement ages of 2181±12 and 1801±21Ma for deformed monzogranitic gneiss and undeformed syenogranite, respectively. The deformed hornblende/biotite-bearing monzogranitic gneisses in the Jiaobei Terrane, along with the Liaoji granitoids, define pre-tectonic granitoid events at ca. 2.2–2.0Ga, whereas the undeformed syenogranite corresponds to the post-tectonic or anorogenic granitoid event at ca. 1.8Ga. Zircons from the pre-tectonic hornblende-bearing monzogranitic gneiss have ɛHf(t) values varying from −2.13 to +6.18 (average=+1.2), two-stage Hf model ages (TDM2) of 2313–2780Ma and same trends of crustal evolution as ca. 2.5Ga TTG rocks, suggesting an origin by remelting of ca. 2.5Ga TTG rocks. Zircons from the pre-tectonic biotite-bearing monzogranitic gneiss have negative ɛHf(t) values of −7.36 to −2.85 (average=−5.26), two-stage Hf model ages (TDM2) of 2930–3206Ma and the same trends of crustal evolution as ca. 2.7 and/or ca. 2.9Ga TTG rocks, indicating derivation by partial melting of ca. 2.7 and/or ca. 2.9Ga TTG rocks. Zircons from the post-tectonic syenogranite display negative ɛHf(t) values ranging from–6.58 to −3.26 (average=−4.89), two-stage Hf model ages (TDM2) varying from 2663 to 2868Ma, and the same trends of crustal evolution as ca. 2.5Ga TTG rocks, indicating an origin by partial melting of ca. 2.5Ga TTG rocks. The linear belt of pre-tectonic, deformed A-type granitoid rocks with emplacement ages of 2.2–2.0Ga from the Liaoji area, Jiaobei Terrane, through to the Bengbu area of Anhui Province, together with the coeval mafic intrusions and bimodal volcanic assemblages, suggest that widespread extensions occurred along the Jiao-Liao-Ji Belt in the period of 2.2–2.0Ga. In contrast, the post-tectonic or anorogenic granitoid events may be linked to extensions following the amalgamation of the North China Craton.

The metacarbonate rocks of Itatuba (Paraíba): A record of sedimentary recycling in a Paleoproterozoic collision zone of the Borborema province, NE Brazil

The Alto Moxotó Terrane (AMT) is a Paleoproterozoic domain structured within the Neoproterozoic collage that forms the Borborema Province in Northeastern Brazil. In the Itatuba region (state of Paraíba) the dominant rocks consist of banded granodioritic–monzogranitic gneisses, augen syenogranitic gneisses and migmatites with limited occurrences of metasedimentary rocks. A metaplutonic–metacarbonate complex appears as concordant intrusive bodies whose emplacement was controlled by contractional shear zones. This complex includes metamafic–ultramafic rocks, leucotonalitic orthogneisses and metacarbonate rocks cut by alkaline orthogneisses emplaced along late transcurrent shear zones. The metamafic–ultramafic complex presents patterns of a tholeiitic magmatic series and exhibits symplectite textures, suggesting that they have reached a high-pressure (eclogite?) metamorphic peak. Uncommon associated rocks are represented by massive and brecciated metacarbonates, which contain abundant inclusions of the country rocks. The structure of this brecciated facies varies from plastic clastomylonitic to a true melting state, confirming the existence of intrusive metacarbonates. Field and petrographic evidence, such as the occurrence of rare rounded pyroxenite inclusions and the association with lamprophyre, suggests the involvement of the lower crust, or even the mantle in their formation. However, the geochemical (rare earth elements (REE) and incompatible elements) and isotopic (C–O) patterns preclude a mantle source for these rocks, confirming that they are remobilized sedimentary carbonates. U–Pb SHRIMP and TIMS zircon studies have been performed on the country and intrusive orthogneisses. In the host orthogneiss, zircon cores yield an age of 2,308±22.5Ma, whereas zircon rims exhibit a cluster with a mean age of 2012±17Ma. For the leucotonalitic orthogneisses that are frequently associated with the rocks of the mafic–ultramafic complex, zircon cores exhibit a spread of ages that may be attributed to the formation of the mafic to felsic complex, and the majority of these ages are centred approximately 2,180Ma. The rims exhibit two clusters of 2,086±19Ma and 1,953±25Ma. The TDM Nd model age of this rock is 2,067Ma; consequently, it is possible that this age interval represents another rock-forming episode associated with the probable eclogite episode. It is therefore likely that the metamafic–ultramafics, leucotonalitic orthogneisses and the metacarbonate rocks represent an Orosirian collisional complex of the AMT. Sm–Nd and U–Pb data from the late alkaline Neoproterozoic transcurrent granites (549, 587, and 621Ma) show reworking of the Paleoproterozoic crust.

Zircon U–Pb–Hf isotopes and whole-rock geochemistry of granitoid gneisses in the Jianping gneissic terrane, Western Liaoning Province: Constraints on the Neoarchean crustal evolution of the North China Craton

The Jianping gneissic terrane (JPGT), Western Liaoning Province (WLP) at the northwestern margin of the Eastern Block of the North China Craton is one of the major Neoarchean high-grade metamorphic terranes. The region is composed dominantly of metamorphosed volcanic and pyroclastic rocks and granitoid gneisses. Geological and petrological features together with geochemical data suggest that the magmatic precursors of these granitoid gneisses are tonalite, trondhjemite, granodiorite (TTG), diorite and monzogranite. Whereas the TTGs show strong gneissic fabric, the monzogranitic rocks are only weakly gneissic and generally display massive structures. LA-ICPMS zircon U–Pb isotopic analyses reveal that the dioritic and TTG gneisses formed at 2506–2532Ma, followed by the emplacement of the monzogranitic gneisses at ∼2496Ma. These granitoid gneisses can be classified into two groups: a low silica group (LSG) and a high silica group (HSG). Detailed petrogenetic studies suggest that the dioritic gneisses (LSG) were derived from the partial melting of an enriched mantle source metasomatized by slab fluids, whereas the tonalitic and granodioritic gneisses of the LSG were produced by the partial melting of subducted oceanic slabs and the melts were contaminated by mantle peridotites during their ascent. In comparison, the HSG granitoid gneisses were generated by the partial melting of the lower continental crust composed mainly of metamorphosed basaltic (garnet amphibolites) and pelitic rocks.Comprehensive zircon U–Pb dating and Lu–Hf isotopic data indicate that the JPGT and the North Chaoyang-Fuxin-Yixian granite-greenstone belt (NCFY-GGB) experienced nearly the same crustal evolution during Neoarchean to early Paleoproterozoic time and major crustal growth at ∼2.5 and ∼2.7Ga was recorded in the WLP. Collectively, the WLP preserves a complex geodynamic history from the mid-ocean ridge, through intra-oceanic arc to Andean-type active continental margin.

U–Pb geochronology and Lu–Hf isotopes of zircons from newly identified Permian–Early Triassic plutons in western Liaoning province along the northern margin of the North China Craton: constraints on petrogenesis and tectonic setting

Mafic to felsic gneisses along the northern margin of the North China Craton (NMNCC), in western Liaoning province, China, were previously assumed to be part of Archean metamorphic basement but are here identified as younger (Permian–Early Triassic) intrusions. LA–ICP–MS zircon U–Pb isotopic dating reveals that the magmatic precursors of the mafic gneisses were emplaced from 295 ± 3 to 259 ± 2 Ma and that the magmatic precursors of the dioritic and monzogranitic gneisses were emplaced at 267 ± 1 and 251 ± 2 Ma, respectively, thus recording a continuum of Permian to Early Triassic magmatism. The mafic and dioritic rocks exhibit zircon eHf(t) values from −20.7 to −3.3, suggesting they were mainly derived from a metasomatized lithospheric mantle source, possibly involving some crustal contamination. The monzogranitic rocks display their zircon eHf(t) values of +0.9 to +4.7, indicating the acidic magma was derived from partial melting of juvenile crustal materials from the depleted mantle source. Crustal model ages (T DM C ) obtained from zircon Hf isotopes of these monzogranitic rocks range from 976 to 1,215 Ma, with an average of 1,074 ± 32 Ma, possibly implying an episode of Grenvillian crustal growth in western Liaoning province. These new lines of evidence show that the NMNCC witnessed abundant magmatic activity and interaction of the crust and mantle during the Permian and Early Triassic and that the mafic magmatism was earlier than the monzogranitic activity. These findings indicate that the monzogranitic activity was the result of underplating of mafic magma with an enriched mantle source. In the context of regional Late Paleozoic to Early Mesozoic magmatic activity, the Permian magmatism occurred in an Andean-style continental margin setting when the Paleo-Asian oceanic plate was subducted beneath the NMNCC, and in this context, the Late Permian to Early Triassic magmatism may have been linked to post-collisional extension and asthenospheric upwelling, suggesting that the western Liaoning province in the NMNCC may be an eastward extension of the Late Paleozoic to Early Mesozoic active continental margin.

Guandishan Granitoids of the Paleoproterozoic Lüliang Metamorphic Complex in the Trans‐North China Orogen: SHRIMP Zircon Ages, Petrogenesis and Tectonic Implications

Abstract:The Paleoproterozoic Lüliang Metamorphic Complex (PLMC) is situated in the middle segment of the western margin of the Trans‐North China Orogen (TNCO), North China Craton (NCC). As the most important lithological assemblages in the southern part of the PLMC, Guandishan granitoids consist of early gneissic tonalities, granodiorites and gneissic monzogranites, and younger gneissic to massive monzogranites. Petrochemical features reveal that the early gneissic tonalities and granodiorites belong to the medium‐K calc‐alkaline series; the early gneissic monzogranites are transitional from high‐K calc‐alkaline to the shoshonite series; the younger gneissic to massive monzogranites belong to the high‐k calc‐alkaline series, and all rocks are characterized by right‐declined REE patterns and negative Nb, Ta, Sr, P, and Ti anomalies in the primitive mantle normalized spidergrams. SHRIMP zircon U–Pb isotopic dating reveals that the early gneissic tonalities and granodiorites formed at ∼2.17 Ga, the early gneissic monzogranites at ∼2.06 Ga, and the younger gneissic to massive monzogranites at ∼1.84 Ga. Sm–Nd isotopic data show that the early gneissic tonalities and granodiorites have ɛNd(t) values of +0.48 to –3.19 with Nd‐depleted mantle model ages (TDM) of 2.76–2.47 Ga, and early gneissic monzogranites have ɛNd(t) values of –0.53 to –2.51 with TDM of 2.61–2.43 Ga, and the younger gneissic monzogranites have ɛNd(t) values of –6.41 to –2.78 with a TDM of 2.69–2.52 Ga. These geochemical and isotopic data indicate that the early gneissic tonalities, granodiorites, and monzogranites were derived from the partial melting of metamorphosed basaltic and pelitic rocks, respectively, in a continental arc setting. The younger gneissic to massive monzogranites were derived by partial melting of metamorphosed greywackes within the continental crust. Combined with previously regional data, we suggest that the Paleoproterozoic granitoid magmatism in the Guandishan granitoids of the PLMC may provide the best geological signature for the complete spectrum of Paleoproterozoic geodynamic processes in the Trans‐North China Orogen from oceanic subduction, through collisional orogenesis, to post‐orogenic extension and uplift.

A comparison of U–Pb and Hf isotopic compositions of detrital zircons from the North and South Liaohe Groups: Constraints on the evolution of the Jiao-Liao-Ji Belt, North China Craton

The Jiao-Liao-Ji orogenic belt is one of the important Paleoproterozoic mobile belts in the North China Craton (NCC) and its central segment consists of the North and South Liaohe Groups and the Liaoji Granitoids. The North and South Liaohe Groups consist of sedimentary and volcanic successions metamorphosed from greenschist to lower amphibolite facies, and the Liaoji granitoids are divisible into 2.2–2.1 Ga (pre-tectonic) monzogranitic gneisses and 1.88–1.85 Ga (post-tectonic) porphyritic monzogranites and alkaline syenites. Combined with cathodoluminescence (CL) imaging, we report LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) U–Pb and Hf isotopic data for detrital zircons from the North and South Liaohe Groups. Zircons grains from both groups have igneous zircon cores that yielded two U–Pb age populations at 2.0–2.2 Ga (major) and ∼2.5 Ga (minor), indicating that their provenances were similarly dominated by Paleoproterozoic Liaoji Granitoids and late Archean basement rocks. Some structureless zircon rims in both groups yield concordant or upper intercept ages ∼1.9 Ga, interpreted as the peak metamorphic age of the two groups. These results suggest that both groups were deposited in the period 2.0–1.9 Ga, shortly after the emplacement of the Liaoji Granitoids. Moreover, zircons from both groups have similar Hf model age peaks ( T DM C peaks at 2.66–2.75 Ga and 2.80–3.00 Ga) and ɛHf values (−5 to +9 and −7 to +5). Based on these striking similarities, we propose that the protoliths of the North and South Groups may have formed simultaneously, and that the basement rocks underneath the two groups belong to the same Archean continental block rather than two different blocks as previously considered. The Archean basement underwent Paleoproterozoic rifting, accompanied by the emplacement of the Liaoji granitoids at 2.2–2.1 Ga and the deposition of the North and South Liaohe Groups at 2.1–1.9 Ga, and the rift was subsequently closed by ∼1.9 Ga.

SHRIMP U–Pb zircon ages of granitoid rocks in the Lüliang Complex: Implications for the accretion and evolution of the Trans-North China Orogen

The Lüliang Complex is situated in the central segment of the Trans-North China Orogen (TNCO), a continent–continent collisional belt along which the discrete Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The complex consists of supracrustal and granitoid rocks, of which the latter can be subdivided into pre-tectonic TTG gneisses, including the Yunzhongshan, Guojiazhuang and Chijianling–Guandishan gneisses; syn-tectonic gneissic granites represented by the Huijiazhuang gneissic granite; and post-tectonic granites, including the Luyashan charnockite, Luchaogou porphyritic granite and Tangershang/Guandishan massive granite. The pre-tectonic TTG gneisses are mostly calc-alkaline and considered to have formed in a magmatic arc environment. SHRIMP U–Pb analyses reveal that the Yunzhongshan gneisses were emplaced at ∼2499 ± 9 Ma, representing the earliest arc-related magmatic event in the Lüliang Complex. This was followed by the intrusion of the Guojiazhuang gneisses at 2375 ± 10 Ma. The most widespread arc-related magmatic event in the region was the emplacement of the Chijianling–Guandishan granitoid gneisses, of which the tonalitic, granodioritic and monzogranitic gneisses were emplaced at 2199 ± 11 Ma, 2180 ± 7 Ma and 2173 ± 7 Ma, respectively. Metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207Pb/ 206Pb age of 1872 ± 7 Ma, consistent with the metamorphic age range of 1880–1820 Ma defined by metamorphic zircons from various high-grade rocks in the TNCO. Thus, the evolution of the Lüliang Complex involved emplacement of the Yunzhongshan granitoids at ∼2499 Ma, the Guajiazhuang granitoids at ∼2375 Ma and the Chijianling–Guandishan granitoids at 2199–2173 Ma, with the final collision between the Eastern and Western Blocks in this area occurring at 1872 ± 7 Ma, the whole series of magmatic events lasting nearly 650 Ma. This suggests that the Trans-North China Orogen represents a long-lived magmatic arc. The new SHRIMP zircon data, combined with structural and petrological studies, also reveal the existence of both syn- and post-collisional granites in the Lüliang Complex. The former is represented by the Huijiazhuang gneissic granite that was emplaced at 1832 ± 11 Ma, whereas the latter include the 1815 ± 5 Ma Luyashan charnockite, 1807 ± 10 Ma Luchaogou porphyritic granite, 1798 ± 11 Ma Guandishan massive granite and 1790 ± 14 Ma Tangershan massive granite.

SHRIMP U–Pb zircon geochronology of the Liaoji granitoids: Constraints on the evolution of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton

The Paleoproterozoic Jiao-Liao-Ji belt lies at the eastern margin of the Eastern Block of the North China Craton and is composed mainly of the Liaohe Group and the Liaoji granitoids. The Liaoji granitoids are divisible into pre-tectonic magnetite and hornblende/biotite monzogranitic gneisses and post-tectonic or anorogenic porphyritic monzogranites and alkaline syenites. SHRIMP U–Pb zircon geochronology, combined with cathodoluminescence (CL) imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the history of the Paleoproterozoic Jiao-Liao-Ji belt. SHRIMP U–Pb zircon analyses reveal that the pre-tectonic magnetite monzogranitic gneisses were emplaced in the period 2176–2166 Ma and metamorphosed at 1914 Ma, and the pre-tectonic hornblende/biotite monzogranitic gneisses were emplaced at 2150–2143 Ma. These data demonstrate that the pre-tectonic monzogranitic gneisses of the Liaoji granitoids were not younger than the deposition of the Liaohe Group, as previously considered, since the latter contains a large amount of 2100–2000 Ma detrital zircons. In contrast, the pre-tectonic monzogranitic gneisses of the Liaoji granitoids or similar-aged granitoids may have been an important component of the provenance for the Liaohe Group. SHRIMP U–Pb zircon analyses also reveal that the post-tectonic porphyritic monzogranites and granites in the Jiao-Liao-Ji belt were emplaced at 1875 ± 10 and 1856 ± 31 Ma, respectively, simultaneously with the emplacement of the alkaline syenites that was dated at 1857 ± 20 and 1843 ± 23 Ma. Similar post-tecotnic or anorogenic granites are not only limited to the Jiao-Liao-Ji belt, but are also found in the North and South Korea, suggesting that the ∼1.85 Ga post-tectonic or anorogenic granitic magmatic event may have occurred in both the Korean Peninsula and the Eastern Block of the North China Craton. The SHRIMP zircon ages of this study combined with lithological, structural, metamorphic and geochemical data is consistent with a rift closure model which suggests that the Archean complexes on the northern and southern sides of the Jiao-Liao-Ji belt were originally situated on a single continental block that underwent Paleoproterozoic rifting, associated with the formation of the Liaohe Group and Liaoji granitoids, and closed upon itself at ∼1.9 Ga.

Geochemical constraints on the petrogenesis of the Proterozoic granitoid gneisses from the eastern segment of the Central Tianshan Tectonic Zone, northwestern China

The Tianshan orogen is divided into the Northern, Central and Southern Tianshan tectonic zones by the northern and southern sutures on both sides of the Central Tianshan Tectonic Zone. The eastern segment of the Central Tianshan Tectonic Zone is characterized by the presence of numerous Precambrian metamorphic blocks and is unconformably overlain by Ordovician–Silurian and late Palaeozoic strata. The Precambrian Kumishi and Pargantag metamorphic blocks are the largest older blocks in the eastern segment of the Central Tianshan Tectonic Zone, consisting mainly of metamorphic granitoids and sedimentary rocks in greenschist to amphibolite facies. There are two major lithological assemblages of the metamorphic granitoids: (1) quartz dioritic gneisses, and (2) granodioritic–monzogranitic gneisses with a minor amount of tonalitic and syenogranitic gneisses in both the Kumishi and Pargantag blocks. The quartz dioritic gneisses are characterized by low Sr/Ce (<5.3) and Sr/Y (<28), relatively high Mg no. (51.0–57.0), K2O (2.65–4.04 wt %) contents and εNd(t) values (−2.37–5.84), and negative Nb and Zr–Hf anomalies, as well as relatively flat chondrite-normalized REE patterns with slightly negative Eu anomalies, suggesting that the quartz dioritic gneisses were derived from partial melting of a depleted mantle source enriched by fluids and sedimentary melts from the subducted slab. However, most of granitic gneiss samples display high K2O contents, low Al2O3/(FeO* + MgO + TiO2) values, and relatively flat chondrite-normalized REE patterns with intensively negative Eu anomalies. Integrated low εNd(t) values and older TDM model ages suggest that crustal materials played a significant role in the petrogenesis of these granitoid gneisses and that they were mainly derived from the partial melting of calc-alkaline mafic to intermediate rocks in the crust. Also, variations in geochemical features between the Kumishi–Gangou and Pargantag regions, such as Zr and Hf, may reflect geographic variability in the development of coeval granitic magmas. Tectonic discrimination for granitoid, using trace elements, together with Nd isotopic data, demonstrates that these granitoid gneisses in the eastern segment of the Central Tianshan Tectonic Zone formed in a continental margin arc during late Mesoproterozoic times.

Geologia, Petrografia e Litogeoquímica dos Gnaisses Porto Alegre, RS, Brasil: Implicações Geotectônicas

The Porto Alegre gneiss is a unit composed of granodioritic to monzogranitic gneisses, with subordinate tonalitic and dioritic rocks. The unit represents the basement intruded by several granitic plutons of Neoproterozoic age. The gneisses show an elongate form of NE-SW direction, and constitute a roof pendant. They have composite banding, which results of injection of dioritic and tonalitic melts, followed by a regional metamorphic event of medium to high grade. Three deformational phases are registered in the gneisses, the latest has a mylonitic nature. Their geochemical features are of medium to high-K, slightly peraluminous granodiorites of calc-alkaline affinity. Comparing the Porto Alegre gneisses with metagranitoids of Pinheiro Machado complex, there are several differences, such as the lower values of Al2O3, Na2O and Sr, and higher values of FeO, MgO, MnO and Rb in the former. These differences indicate that there is no correlation between the Porto Alegre gneisses and Pinheiro Machado complex.

A review of Archaean and Paleoproterozoic evolution of the In Ouzzal granulitic terrane (Western Hoggar, Algeria)

The In Ouzzal terrane (Western Hoggar) is an example of Archaean crust remobilized during a very-high-temperature metamorphism related to the Paleoproterozoic orogeny (2 Ga). Pan-African events (≈0.6 Ga) are localized and generally of low intensity. The In Ouzzal terrane is composed of two Archaean units, a lower crustal unit made up essentially of enderbites and charnockites, and a supracrustal unit of quartzites, banded iron formations, marbles, Al–Mg and Al–Fe granulites commonly associated with mafic (metanorites and garnet pyroxenites) and ultramafic (pyroxenites, lherzolites and harzburgites) lenses. Cordierite-bearing monzogranitic gneisses and anorthosites occur also in this unit. The continental crust represented by the granulitic unit of In Ouzzal was formed during various orogenic reworking events spread between 3200 and 2000 Ma. The formation of a continental crust made up of tonalites and trondhjemites took place between ⩾3200 and 2700 Ma. Towards 2650 Ma, extension-related alkali-granites were emplaced. The deposition of the metasedimentary protoliths between 2700 and 2650 Ma, was coeval with rifting. The metasedimentary rocks such as quartzites and Al–Mg pelites anomalously rich in Cr and Ni, are interpreted as a mixture between an immature component resulting from the erosion and hydrothermal alteration of mafic to ultramafic materials, and a granitic mature component. The youngest Archaean igneous event at 2500 Ma includes calc-alkaline granites resulting from partial melting of a predominantly tonalitic continental crust. These granites were subsequently converted into charnockitic orthogneisses. This indicates crustal thickening or heating, and probably late Archaean high-grade metamorphism coeval with the development of domes and basins. The Paleoproterozoic deformation consists essentially of a re-activation of the pre-existing Archaean structures. The structural features observed at the base of the crust argue in favour of deformation under granulite-facies. These features are compatible with homogeneous horizontal shortening of overall NW–SE trend that accentuated the vertical stretching and flattening of old structures in the form of basins and domes. This shortening was accommodated by horizontal displacements along transpressive shear corridors. Reactional textures and the development of parageneses during the Paleoproterozoic suggest a clockwise P–T path characterized by prograde evolution at high pressures (800–1050 °C at 10–11 kbar), leading to the appearance of exceptional parageneses with corundum–quartz, sapphirine–quartz and sapphirine–spinel–quartz. This was followed by an isothermal decompression (9–5 kbar). Despite the high temperatures attained, the dehydrated continental crust did not undergo any significant partial melting. The P–T path followed by the granulites is compatible with a continental collision, followed by delamination of the lithosphere and uprise of the asthenosphere. During exhumation of this chain, the shear zones controlled the emplacement of carbonatites associated with fenites.

Age, petrogenesis and significance of 1 Ga granitoids and related rocks from the Sendra area, Aravalli Craton, NW India

We present new geochronological, petrological, geochemical and isotopic data for granitic and related rocks from the Aravalli Craton, Rajasthan, northwestern India. In the Sendra area, five variably deformed granitoid plutons, ranging in composition from tonalite to granite, cut across carbonate-rich metasedimentary rocks of the Delhi Supergroup. The largest of these bodies, the Chang pluton (∼15 km 2) is dominated by monzogranitic gneisses and aplitic dykes, composed of subequal proportions of quartz, plagioclase (An 7–20) and microcline (Or 92–98), with lesser biotite (Fe ∗=0.8–0.9) and accessory muscovite (Fe ∗=0.7–0.8). U–Pb zircon data (TIMS method) for a biotite granite gneiss yield a weighted mean 207Pb/ 206Pb age of 967.8±1.2 Ma, which we interpret as representing the time of magmatic crystallization. Rb–Sr whole-rock isotopic data for the Chang pluton, including new analyses as well as previously published ones, yield a regression of 906±67 Ma (MSWD=82), which is barely within error of the U–Pb age. There is evidence for open-system behaviour in the Rb–Sr system, particularly for whole-rock samples with low Sr concentrations, and consequently high Rb/Sr. Sm–Nd isotopic data fail to yield meaningful age information. Initial isotopic ratios (at 968 Ma) for Chang pluton granitoids ( I Sr=0.7110±14; ε Nd=−3.28±0.47) are compatible with source materials similar to Archaean amphibolitic rocks of the Banded Gneiss Complex. Spatially associated with the Chang pluton is a massive metagabbro, composed of plagioclase (An 45–68) and magnesio-hornblende (Fe ∗=0.3–0.4), with secondary Cl-rich scapolite and ferrian zoisite. The scapolite and zoisite likely crystallized from metamorphic fluids that interacted with nearby calc-silicate schists and gneisses of the Delhi Supergroup. Aside from slight enrichments in Rb, U, Th and Ba, this metagabbro retains a primitive chemical signature similar to N-MORB (LREE depletion, low K), and initial isotopic ratios ( I Sr=0.7058; ε Nd=+2.9) that approach model depleted mantle at 968 Ma. The metagabbro is chemically and isotopically similar to mafic metavolcanic and related rocks that have been considered to represent ocean-floor and island-arc basaltic magmas. All available data are compatible with the idea that these rocks represent the products of convergent margin processes during the Early Neoproterozoic, a conclusion that could have important implications for the construction of the Rodinia supercontinent.

Major and trace element geochemistry and isotope (Sr, Nd, Pb, O) systematics of an Archaean basement involved in a 2.0 Ga very high‐temperature (1000°C) metamorphic event: In Ouzzal Massif, Hoggar, Algeria

ABSTRACTThe In Ouzzal granulitic massif is composed mainly of various meta‐igneous rocks which, in spite of Rb, U, Th, Cs and some K and Sr mobility, can be dated and generally classified according to their chemical composition as follows.Basic and ultrabasic granulites interlayered with the metasediments correspond to (1) ultrabasic cumulates from dislocated tholeiitic bodies, (2) ancient komatiitic to high‐Mg tholeiitic basalts similar to the suites found in Archaean greenstone belts and (3) calcalkaline protoliths of high‐K andesitic composition. No geochronological constraints are available apart from the depositional age of some associated sediments which is younger than 2.70 Ga detrital zircons, and the Nd model age of the andesitic granulites of c. 3.4 Ga.In spite of the high‐grade metamorphism, the acidic magmatic precursors of the charnockites can be divided in three groups. (1) The most juvenile acid orthogneisses are trondhjemitic or tonalitic in composition, being similar to the TTG suites which are classically considered to be formed by partial melting of mantle‐derived protoliths. The 3.3–3.2 Ga TDM indicates a possible age of separation from the mantle reservoir while the plutons may have been emplaced between 3.3 and 2.7 Ga (U–Pb zircon & Nd ages). (2) A group of alkaline granitic gneisses, similar in composition to rift‐related‐granites, were emplaced at 2650±10 Ma (U–Pb & Rb–Sr ages) in a thick continental crust. (3) Calcalkaline granodioritic and monzogranitic suites derived from the partial melting of continental precursors (3.5–3.3 Ga), in lower to middle levels of the continental crust. They were emplaced close to 2.5 Ga during crustal thickening.The very high‐temperature metamorphism occurred at 2002±7 Ma from the age of synfoliation intrusions and was probably related to major overthrusting. Retrogressive metamorphism is dated at 1.95 Ga from garnet‐Nd ages. In spite of the very high‐temperature conditions, partial melting during granulite facies metamorphism may be restricted to scarce cordierite‐bearing monzogranitic gneisses. The 2.0 Ga VHT metamorphism could be related to overthrusting, extensional or underplating processes.