Trondhjemite–granodiorite Gneisses Research Articles

Episodic crustal growth in the southern segment of the Trans-North China Orogen across the Archean-Proterozoic boundary

The Dengfeng and Taihua Complexes are well-exposed Neoarchean to Paleoproterozoic units in the southern segment of the Trans-North China Orogen (TNCO). Zircon U–Pb dating shows that the Dengfeng Complex records two episodes (2568±11Ma and 2306±6Ma) of tonalite–trondhjemite–granodiorite (TTG) magmatism. All of the TTG rocks are characterized by high SiO2 (66.7–75.4wt%), Na2O (3.20–5.06wt%) and relatively low MgO (0.40–1.78wt%). The Late Neoarchean TTG gneisses have very low contents of HREE (YbN=0.69–2.75) and Y (1.73–7.07ppm), with moderate [La/Yb]N (24.1–53.8) and high Sr/Y (65.1–291.3) ratios. The Early Paleoproterozoic TTG gneisses have low contents of HREE (YbN=2.93–6.37) and Y (6.7–11.0ppm), with moderate [La/Yb]N (10.1–27.0) and Sr/Y (10.6–52.1) ratios. Both suites show pronounced negative Nb–Ta, P and Ti anomalies but positive Sr and Pb anomalies. The Late Neoarchean TTG gneisses all have similar bulk-rock Nd and zircon Hf model ages with mainly positive ɛNd(t), and are interpreted as resulting from the melting of dominantly juvenile thickened lower crust with residual garnet and amphibole. The Early Paleoproterozoic TTG gneisses have extremely variable ɛNd(t) (−6.23 to +4.23) and heterogeneous zircon ɛHf(t) (−3.3 to +3.1), which are also best interpreted as resulting from the partial melting of thickened lower crust with residual amphibole and garnet. The Taihua Complex in the Xiaoqinling area records three episodes of Early Paleoproterozoic TTG magmatism (2.48Ga at Caotan, 2.31Ga at Houjiacun and 2.16Ga at Bayuan), younger than the Taihua Complex in the Lushan area (2.85–2.72Ga). All rocks have relatively low contents of HREE (YbN=1.03–8.32) and Y (2.84–24ppm), with moderate [La/Yb]N (8.7–88.4) and Sr/Y (19.8–125.8) ratios, and show negative Ta–Nb and Ti anomalies and positive Sr and Pb anomalies. The Caotan gneisses at 2.48Ga and the Houjiacun TTG gneisses at 2.31Ga have low Mg# (0.14–0.45), low Cr (<42ppm) and Ni contents (1–21ppm), with variable but overall positive ɛNd(t) and ɛHf(t) values, and were derived from the partial melting of thickened lower crust with residual garnet and amphibole. The younger Bayuan TTG gneisses at 2.16Ga have low SiO2 (57.11–64.89wt%), high MgO (2.64–4.62wt%), Cr (100–247ppm) and Ni (32–80ppm), with negative whole rock ɛNd(t) and zircon ɛHf(t) values, resulted from the partial melting of delaminated lower crust that interacted with peridotitic mantle.The geochronology of the Dengfeng Complex (in the Dengfeng area) and the Taihua Complex (in the Lushan, Xiong’er and Xiaoqinling areas) reveals at least four magmatic episodes in the southern segment of the TNCO from the Late Mesoarchean to Early Paleoproterozoic (2.85–2.72Ga, 2.57–2.48Ga, 2.34–2.30Ga and 2.20–2.07Ga). The rocks of the two early episodes are dominantly of juvenile compositions with mostly positive whole rock ɛNd(t) and zircon ɛHf(t) values, suggesting two episodes of crustal growth formed in a subduction tectonic setting. The magmatic rocks of the third episode consist of both the juvenile and pre-existing crustal materials with variable whole rock ɛNd(t) and zircon ɛHf(t) values, which were generated in a subduction zone during the initial assembly of the NCC within the Columbia supercontinent cycle. The final episode of magmatism lacks juvenile materials with whole rock ɛNd(t) and zircon ɛHf(t) values being consistently negative. These may have resulted from the orogenic collapse. The episodic continental growth recorded in the southern segment of the TNCO was caused by subduction and consequent orogeny, consistent with global supercontinent cycles within the Late Archean and Early Paleoproterozoic.

Petrogenesis, P–T–t path, and tectonic significance of high-pressure mafic granulites from the Jiaobei terrane, North China Craton

High-pressure (HP) mafic granulites in the Jiaobei terrane are composed predominantly of garnet mafic granulites, garnet–hypersthene granulites, and garnet amphibolites, and they are found as irregular lenses or deformed dike swarms within tonalitic–trondhjemitic–granodioritic gneisses and granitic gneisses. The HP mafic granulites contain four distinct metamorphic assemblages, of which the early prograde assemblage (M1) is represented by the cores of garnets, together with mineral inclusions of clinopyxene+plagioclase±quartz, and it formed at 740–770°C and 0.90–1.00GPa. In contrast, the peak assemblage (M2) consists of high-Ca cores in garnet, high-Al cores in clinopyroxene, and high-Na cores in plagioclase in the matrix, which formed under P–T conditions of 850–880°C and 1.45–1.65GPa. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of orthopyroxene+clinopyxene+plagioclase±quartz±amphibole±magnetitie symplectites or coronas surrounding some garnet grains, with P–T conditions of 780–830°C and 0.65–0.85GPa. Surrounding some garnet grains are symplectites of amphibole+plagioclase+quartz±magnetitie, which formed during a cooling retrograde stage (M4) with P–T conditions of 590–650°C and 0.62–0.82GPa. An integrated study involving laser Raman analysis of mineral inclusions, cathodoluminescence imaging, and in situ U–Pb dating of zircons shows that the protolith ages of the HP mafic granulites are mainly 2550–2500Ma, and that the timing of the peak metamorphism of the HP mafic granulites ranges from 1900 to 1860 Ma, as recorded by the cores of metamorphic zircons. On the other hand, the medium- to low-pressure granulite–amphibolite facies retrogression occurred mainly at 1860–1820Ma, as recorded by the rims of some single zircon grains and the zircon grains that contain inclusions of garnet+orthopyroxene+plagioclase+sphene. The combination of petrography, mineral compositions, metamorphic reaction history, thermobarometry, and geochronology defines a near-isothermal decompressional clockwise P–T–t path for the Jiaobei HP mafic granulites, suggesting that the Jiaobei terrane underwent initial crustal thickening during 1950–1860Ma, followed by relatively rapid exhumation, cooling, and retrogression in the period 1860–1820Ma. This tectonothermal path was probably generated by subduction and collision-related tectonic processes.

Field and petrographic evidence for partial melting of TTG gneisses from the central region of the mainland Lewisian complex, NW Scotland

The central region of the mainland Lewisian complex is dominated by granulite-facies tonalite–trondhjemite–granodiorite (TTG) gneisses that are highly depleted in some mobile trace elements (Cs, Rb, Th and U) relative to amphibolite-facies TTG gneisses elsewhere in the Lewisian complex and to the average composition of TTG gneisses worldwide. Over almost half a century of research there has been vigorous debate as to the origin of this depletion, in particular with respect to the role of partial melting and melt loss. Here we provide field and petrographic evidence that TTG gneisses across the central region partially melted during granulite-facies (Badcallian) metamorphism. Partial melting occurred largely by fluid-absent incongruent reactions consuming plagioclase, quartz, hornblende and biotite to produce melt and peritectic clino- and orthopyroxene. The preservation of dry, granulite-facies assemblages requires loss of melt, consistent with the presence of an interconnected network of leucosomes and larger felsic sheets that probably record segregation and transfer of melt to higher crustal levels. Regardless of whether or not partial melting and melt loss can explain fully the unusual geochemical signature of the central region TTG gneisses, these fundamental processes did occur. Supplementary material: Figure S1, showing additional field photographs of boulders from Poll Eòrna, is available at www.geolsoc.org.uk/SUP18580 .

The 3.1 Ga Nuggihalli chromite deposits, Western Dharwar craton (India): Geochemical and isotopic constraints on mantle sources, crustal evolution and implications for supercontinent formation and ore mineralization

Nuggihalli greenstone belt is one of the oldest greenstone belts (3.4–3.0Ga) in the Western Dharwar craton, southern India. It consists of conformable metavolcanic (e.g., komatiite and komatiitic basalt) and metasedimentary rocks belonging to the Sargur Group. Sill-like ultramafic–mafic plutonic bodies are present within these schistose rocks which are in turn enclosed by tonalite–trondhjemite–granodiorite gneisses (TTG). The plutonic suite occurs as a layered succession of serpentinite (after dunite) and tremolite–chlorite–actinolite schist (after peridotite) hosting chromitite bodies, anorthosite, pyroxenite, and gabbro hosting magnetite bands. Whole-rock Sm–Nd data for the peridotite–anorthosite–pyroxenite–gabbro unit yield a correlation line with a slope corresponding to an age of 3125±120Ma (MSWD=1.3) which is similar to ages of komatiitic rocks of the older greenstone belts in the craton. A whole rock Pb–Pb errorchron age of 2801±110Ma (MSWD=102) has been obtained for the entire plutonic ultramafic–mafic suite; this represents (partial) redistribution/resetting of the U–Pb system during a younger metamorphic event and by the magmatic activity during formation of the younger greenstone belts. The positive εNd values (+1.7 to +3.4) of the ultramafic–mafic rocks, and low initial 87Sr/86Sr values (at 3.1Ga) of the gabbros (0.70097–0.70111) implies derivation of the parental magma from a depleted mantle source. The REE pattern of the metavolcanic schists bears resemblance with the pattern of Al-depleted komatiites. Major and trace element variation in the schists correspond with the fractionation trend exhibited by komatiites to komatiitic basalts in the older greenstone belts within the craton. Coherent patterns of whole-rock major and trace element data, along with the layered nature of the sill-like ultramafic–mafic rocks indicate that the plutonic and volcanic suites are related by analogous fractional crystallization processes. Comparison of our age data with global plutonic and volcanic ultramafic–mafic rock occurrences in greenstone belts supports an increase in komatiitic activity from 3.5Ga to 2.7Ga, which is most likely related to a supercontinent cycle. High-Mg magmas such as komatiites and their plutonic equivalents host important metal deposits like chromite, Ni-sulfide and minor PGE mineralization. The uneven distribution of metal deposits over time can be explained by supercontinent cycles. The 3.1Ga chromite deposits of the Nuggihalli greenstone belt are perhaps related to the amalgamation stage of a supercontinent.

Precambrian geology of China

Precambrian geology of China

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.

Role of H2O in the formation of garnet coronas during near‐isobaric cooling of mafic granulites: the Tasiusarsuaq terrane, southern West Greenland

AbstractThe Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite–trondhjemite–granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium‐pressure granulite facies conditions during M1A in the Neoarchean, resulting in the development of two‐pyroxene melanosome assemblages in mafic granulites containing garnet‐bearing leucosome. Reworking of these rocks during retrogression introduced garnet to the melanosome in the form of overgrowths, coronas and grain necklaces that separate the mafic minerals from plagioclase. NCFMASHTO pseudosection modelling constrains the peak metamorphism during M1A to ∼850 °C and 7.5 kbar at fluid‐saturated conditions. Following M1A, the rocks retained their M1A H2O content and became fluid‐undersaturated as they underwent near‐isobaric cooling to ∼700 °C and 6.5–7 kbar, prior to reworking during M1B. These low H2O contents allowed for the formation of garnet overgrowths and coronas during M1B. The stability of garnet is greatly increased to lower pressure and temperature in fluid‐absent, fluid‐undersaturated mafic rocks, indicating that fluid and melt loss during initial granulite facies metamorphism is essential for the introduction of garnet, and the formation of garnet coronas, during retrogression. The occurrence of garnet coronas is consistent with, but not unique to, near‐isobaric cooling paths.

Zircon U–Pb ages and geochemistry of the Huai’an TTG gneisses terrane: Petrogenesis and implications for ∼2.5 Ga crustal growth in the North China Craton

The Huai’an gneiss terrane is a typical Archean terrane within the Trans-North China Orogen of the North China Craton (NCC) and is composed of Tonalite–Trondhjemite–Granodiorite (TTG) suite and dioritic granulite facies gneisses. Precise SHRIMPII and Cameca IMS1280 zircon U–Pb ages demonstrate that the Huai’an gneisses formed around 2.55–2.50Ga and underwent granulite facies metamorphism at 1.85–1.80Ga. The TTG gneisses are compositionally equivalent to high-SiO2 adakites, with high SiO2 and Al2O3 concentrations, strongly fractionated rare earth element (REE) patterns ((La/Yb)N=10–54) with positive or negligible Eu anomalies (Eu/Eu*=0.9–3.4), low heavy REE (HREE) and Y concentrations, high Sr (435–938ppm) concentrations, and high Sr/Y ratios (48–261). High Mg numbers (33–60) and high compatible element concentrations (e.g., Cr=11–183ppm and Ni=11–82ppm) indicate that the magmas that formed these gneisses were derived from interaction between adakitic melts and mantle material. The dioritic gneisses have compositions similar to high-Mg diorites or sanukitoids, with high Al2O3 (14.46–17.61wt%), MgO (3.0–6.4wt%), Cr (100–304ppm), and Ni (57–109ppm) concentrations, and fractionated REE patterns with weak negative or no Eu anomalies (Eu/Eu*=0.8–1.0). All the TTG and dioritic gneiss samples analysed during this study have negative Nb, Ta, and Ti anomalies, compared to neighbouring elements in spider-diagrams, with the majority having high positive ɛNd(t) and ɛHf(t) values, close to values of the depleted mantle. These geochemical characteristics strongly suggest that protoliths of the Huai’an TTG and dioritic gneisses originated in a juvenile crust-forming event at 2.55–2.50Ga through partial melting of subducted oceanic crustal material; these magmas also assimilated mantle peridotite material during their ascent. This indicates that ∼2.5Ga mantle-derived magmatism led to significant crustal growth in the North China Craton, and was probably associated with subduction.

Crustal growth and intracrustal recycling in the middle segment of the Trans-North China Orogen, North China Craton: a case study of the Fuping Complex

AbstractThe most important crustal growth on Earth occurred at ~2.7 Ga, but the North China Craton (NCC) is characterized by prevalent development of ~2.5 Ga juvenile crust, with relatively rare records of ~2.7 Ga crustal growth. The Fuping Complex in the middle segment of the Trans-North China Orogen (TNCO) between the Eastern and Western blocks of the NCC is composed mainly of ~2.5 Ga Fuping tonalitic–trondhjemitic–granodioritic (TTG) gneisses and Longquanguan augen gneisses, ~2.1 Ga Nanying granitic gneisses and the Wanzi supracrustal rocks. Previous studies have suggested one major phase of crustal growth at ~2.5 Ga, possible intracrustal recycling at ~2.1 Ga and the presence of older rocks in the Fuping Complex, but there has been no record of ~2.7 Ga crustal growth. The Fuping TTG gneisses are dominated by stromatic migmatite, and new U–Pb dating of magmatic zircons from two stromatic migmatite samples yielded three different ages: (1) 2.75 Ga, which is the oldest age obtained from the Fuping TTG gneisses, (2) 2.54 Ga, which just falls in the published zircon U–Pb age range of 2.53 to 2.47 Ga for the Fuping TTG gneisses, and (3) 2.11 Ga, which is almost the same as the age of the Nanying granitic gneisses. Therefore, there are two generations of TTG gneisses in the Fuping Complex. Importantly, both of the 2.75 and 2.54 Ga zircons have the highest εHf(t) values, almost equal to the contemporaneous depleted mantle. This indicates high contributions of juvenile material to the two generations of TTG gneisses. In contrast, the 2.11 Ga zircons have apparently low εHf(t) values of −0.47 to +2.04, just falling in between 2.55 and 2.75 Ga continental crust values. This strongly suggests the reworking of the two generations of TTG gneisses at 2.1 Ga. Zircon U–Pb and Hf isotopes convincingly reveal two major phases of crustal growth in the Fuping Complex at ~2.7 and ~2.5 Ga, the same as in the northern and southern segments of the TNCO, and also confirm one major phase of intracrustal recycling at ~2.1 Ga, which may be responsible for the Nanying granitic gneisses.

Geochemical and Nd isotope constraints on petrogenesis of granitoids from NW part of the eastern Dharwar craton: Possible implications for late Archaean crustal accretion

Geochemical and Nd isotope data on granitoids of the NW part of the late Archaean eastern Dharwar craton are presented to elucidate their petrogenesis and role in crust formation. The granitoids are divided into three suites viz. trondhjemite–granodiorite gneisses, biotite monzogranites and porphyritic biotite granodiorites. The gneisses are pre- to syn-kinematic (with respect to deformation in the adjacent Hungund–Kushtagi schist belt), which show variable SiO 2 and Al 2O 3, enriched LREE and depleted HREE with slightly negative to no Eu anomalies. They display unusual chemistry in having higher FeO(T), K 2O, Ba, Cr and Ni compared to the typical Archaean tonalite–trondhjemite–granodiorite (TTG). The biotite monzogranites are mostly syn- to late-kinematic and exhibit evolved calc-alkaline compositions with high SiO 2, K 2O, LILE and LREE, depleted to undepleted HREE and strongly negative to no Eu anomalies. The porphyritic granodiorites show syn- to late-kinematic calc-alkaline, sanukitoid-like character with a wide range of SiO 2, higher TiO 2, P 2O 5, Sr, Ba, Cr and Ni, and lower Rb. They, however, uniquely display higher K 2O, ΣREE and Th than typical sanukitoids. The trondhjemite–granodiorite gneisses are interpreted as product of melting of a subducted basaltic slab followed by slight contamination from the overlying metasomatized mantle wedge. Subsequent melting of the extremely metasomatized mantle wedge resulted in formation of the parental magma of the porphyritic granodiorites. Intrusion of the latter triggered melting of the TTG crust accreted earlier and generated the evolved monzogranites. The monzogranites occurring east of the Hungund–Kushtagi schist belt show higher εNd but lower T DM ages than those occurring to the west, indicating that terranes with different histories were juxtaposed by lateral accretion.

Association of Neoproterozoic A- and I-type granites in South China: Implications for generation of A-type granites in a subduction-related environment

Neoproterozoic magmatism in the Yangtze Block of South China produced voluminous S- and I-type granites, and sparse A-type granites. The Daxiangling A-type granitic pluton is spatially associated with the Shimian I-type pluton at the western margin of the Yangtze Block. Both plutons have similar SHRIMP zircon U–Pb ages of ∼ 800 Ma and are slightly younger than the tonalite–trondhjemite–granodiorite (TTG) gneisses in the area. The Shimian pluton is composed of granite and monzogranite with high SiO 2 (69.3–76.6 wt.%), Na 2O (2.79–3.80 wt.%) and K 2O (3.94–5.87 wt.%), and low Fe 2O 3 (0.96–3.06 wt.%) and MgO (0.12–0.50 wt.%). The Daxiangling pluton consists of alkali–feldspar granites with higher SiO 2 (76.3–79.3 wt.%) and lower Al 2O 3 (10.6–11.9 wt.%) and CaO (0.21–0.55 wt.%) than the Shimian granites. Both plutons are slightly peraluminous (A/CNK = 1.00–1.12) and belong to the high-K, calc-alkaline series. The Daxiangling alkali-feldspar granites have much higher Zr, Hf, Ga and HREE, and lower Sr than the Shimian granites. On the chondrite-normalized REE diagram, both plutons are enriched in LREE, but the rocks of the Daxiangling pluton show relatively flat patterns because of their higher HREE contents. Their primitive-mantle normalized spidergrams display negative Eu, Nb, Ta, Sr, P and Ti anomalies. Both plutons have nearly identical whole-rock Nd and zircon Hf isotopic compositions ( ε Nd( t) = + 1 and ε Hf( t) = + 5 to + 9), similar to the TTG gneisses. We suggest that the I-type granites of the Shimian pluton were produced by dehydration melting of the TTG rocks as a result of underplating of mantle-derived mafic magmas. The rocks above the I-type magma source may have been converted to charnockites by heating and dehydration. With increasing temperatures, the charnockites underwent partial melting at temperatures > 900 °C to produce A-type magmas. Both the A- and I-type granites formed at an extensional active continental margin, perhaps in a back-arc environment. A-type granites with mantle-like isotopic signatures may be reworked or recycled from juvenile crustal rocks. The association of A- and I-type granites suggests that aluminous A-type granite is most probably derived from a charnockite source heated by large-scale magmatic underplating, rather than a tonalite source.

Mantle wedge involvement in the petrogenesis of Archaean grey gneisses in West Greenland

The Archaean crust in West Greenland is dominated by grey orthogneiss complexes formed in periods of crustal accretion at around 3.8, 3.6, 3.2, 3.0–2.9 and 2.8–2.7 Ga. The majority of the gneisses have tonalite–trondhjemite–granodiorite (TTG) compositions, while subordinate quartz–dioritic and dioritic gneisses have calc-alkaline compositions. The major and trace element chemistry of gneiss samples has been compiled from three large regions representing different terranes and ages in southern and central West Greenland, the Godthåbsfjord, Fiskefjord and Disko Bugt regions. The TTG gneisses are typical for their kind and show little variation, except marked Sr enrichment in the Fiskefjord area and slight Cr enrichment in a unit within the Disko Bugt region. Thus, while most of the crust has probably formed from magmas derived by slab melting, local involvement of mantle-derived components is suggested. Most of the diorites have geochemical signatures compatible with mantle-derived parental magmas, i.e., elevated Mg, Cr and flat chondrite-normalised REE patterns. A group of quartz–diorite and diorite samples from the Fiskefjord region exhibits marked enrichment in Sr, Ba, P, K and REE, combined with steep REE patterns. A similar but much more pronounced enrichment in the same elements characterises Palaeoproterozoic subduction-related monzodiorites within the Nagssugtoqidian orogen, as well as carbonatites and carbonatitic lamprophyres within the same part of West Greenland. We argue that the parental magmas of the enriched diorites are derived by partial melting from regions within the mantle that have been metasomatised by carbonatite-related material, e.g., in the form of carbonate–apatite–phlogopite veins. Alternatively, ascending slab melts may have reacted with carbonatite-metasomatised mantle. Carbonatitic carbonates have high Sr and Ba, and carbonatitic apatite has high P 2O 5 and very steep REE spectra. Adding such a component to a peridotite-derived magma produces geochemical features similar to those of sanukitoids, except that high phosphorus is not described as typical of sanukitoids. We observe that the enriched diorites from Greenland are sanukitoid-like, although they are not sanukitoids by the original definition, and their genesis requires a twist to the current models for sanukitoid petrogenesis.

New light on the construction, evolution and correlation of the Langavat Belt (Lewisian Complex), Outer Hebrides, Scotland: field, petrographic and geochronological evidence for an early Proterozoic imbricate zone

The South Harris Complex is a domain of largely Palaeoproterozoic rocks within the late Archaean, tonalite–trondhjemite–granodiorite (TTG)-dominated Lewisian gneisses of the Outer Hebrides, NW Scotland. The complex is distinguished by a high proportion of metasedimentary rocks and distinctive meta-igneous units, in part representing the remnants of a continental volcanic arc. The Langavat Belt defines the NE border zone of the South Harris Complex, separating the latter from the Archaean gneisses further NE, and has been repeatedly interpreted as a discrete supracrustal unit. However, detailed mapping and petrographic analysis reveals that up to 60% of the belt may be composed of highly deformed felsic orthogneiss. The metasedimentary rocks that are present have a late Archaean zircon provenance signature, implying that they are younger than, and possibly derived from the TTG gneisses to the NE. Previous work on zircon populations in the Leverburgh metasediments, towards the southern flank of the South Harris Complex, indicates very different provenance and we repudiate correlation of the two sequences. The disposition of units of metasediment, amphibolite and felsic orthogneiss is not a primary, pre-tectonic feature and is very unlikely to be the result of folding. Although direct evidence in the form of early fabrics has been wiped out by later, penetrative ductile shearing and metamorphic annealing, we propose that the Langavat Belt was assembled as a zone of thrust imbrication during early Proterozoic contraction. After imbrication, ductile shearing occurred in two distinct periods, separated by intrusion of granite pegmatites dated at c . 1660 Ma.

Archean geodynamics in the Central Zone, North China Craton: constraints from geochemistry of two contrasting series of granitoids in the Fuping and Wutai complexes

Abstract The Archean to Paleoproterozoic Central Zone of the North China Craton is situated between the Eastern and Western Archean continental blocks and contains two contrasting series of Neoarchean granitoids: the 2523–2486 Ma tonalite−trondhjemite–granodiorite (TTG) gneisses in the Fuping Complex, and the 2555–2525 Ma calc-alkaline granitoids (tonalite, granodiorite, granite and monzogranite) in the Wutai Complex. The Fuping TTG gneisses most likely formed from partial melting of ∼2.7 Ga basalts at >50 km, with an involvement of ∼3.0 Ga crustal material. The Wutai granitoids have higher K 2 O, LILE and Rb/Sr, but lower Sr/Y and La N /Yb N than the Fuping TTG gneisses, are characterized by Nd T DM from 2.5 to 2.8 Ga and e Nd( t ) from 0.49 to 3.34, and are derived from partial melting of a juvenile source at The geochemistry of these two contrasting series of Neoarchean granitoids provides further evidence that the Wutai Complex originated and evolved separately from the Fuping Complex. The Wutai Complex most likely formed as an oceanic island arc with volcanism and synvolcanic granitoid intrusions at 2555–2525 Ma. The Wutai Complex was subsequently accreted onto the Eastern Archean Continental Block, and was probably responsible for crustal thickening and TTG magmatism at 2523–2486 Ma in the Fuping Complex (as part of the Taihangshan–Hengshan block), at the western margin of the Eastern Archean Continental Block.

High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications

Both highand medium-pressure granulites have been found as because of the absence of modal minerals. The combination of petrographic textures, mineral compositions, metamorphic reaction enclaves and boudins in tonalitic–trondhjemitic–granodioritic gneisses in the Hengshan Complex. Petrological evidence from these history, petrogenetic grids and thermobarometric data defines a nearrocks indicates four distinct metamorphic assemblages. The early isothermal decompressional clockwise P–T path for the Hengshan prograde assemblage (M1) is preserved only in the high-pressure granulites, suggesting that the Hengshan Complex underwent initial granulites and represented by quartz and rutile inclusions within crustal thickening, subsequent exhumation, and cooling and retrothe cores of garnet porphyroblasts, and omphacite pseudomorphs gression. This tectonothermal path is considered to record a major that are indicated by clinopyroxene + sodic plagioclase symplectic phase of collision between two continental blocks, which resulted in intergrowths. The peak assemblage (M2) consists of clinopyroxene the final assembly of the North China Craton at >1·8 Ga. + garnet + sodic plagioclase + quartz ± hornblende in the high-pressure granulites and orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the medium-pressure granulites.

Archean blocks and their boundaries in the North China Craton: lithological, geochemical, structural and P– T path constraints and tectonic evolution

An examination of lithological, geochemical, geochronological, structural and metamorphic P– T path data suggests that the basement of the North China Craton can be divided into Eastern and Western Blocks, separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone, called the Trans-North China Orogen. The Eastern Block consists predominantly of Late Archean domiform tonalitic–trondhjemitic–granodioritic (TTG) batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at ∼2.5 Ga, with anticlockwise P– T paths. Some Early to Middle Archean rocks are locally present in the Eastern Block, but their tectonic history is unclear due to reworking by the 2.5 Ga tectonothermal event. The Western Block has a Late Archean assemblage, structural style and metamorphic history similar to that of the Eastern Block, but it differs in the absence of early to middle Archean assemblages and in being overlain by and interleaved with Paleoproterozoic khondalites, which were affected by a ∼1.8 Ga metamorphic event involving clockwise P– T paths. A mantle plume model is proposed for the formation and evolution of Late Archean basement rocks in the Eastern and Western Blocks based on a combination of extensive exposure of TTG gneisses, affinities of mafic rocks to continental tholeiitic basalts, presence of voluminous komatiitic rocks, dominant diaprism-related domiform structures, anticlockwise P– T paths, and a short time span from the primary emplacement of TTG and ultramafic to mafic rocks until the onset of regional metamorphism. Between the two blocks is the Trans-North China Orogen which is bounded by two major fault systems and is composed of Late Archean to Paleoproterozoic TTG gneisses and granitoids, interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra-arc basin environments. These rocks underwent multiple phases of compressional deformation and peak high-pressure metamorphism followed by rapid exhumation during the Late Paleoproterozoic at ∼1.8 Ga as a result of collision between the Eastern and Western Blocks, resulting in the amalgamation of the North China Craton.

Tectonothermal history of the basement rocks in the western zone of the North China Craton and its tectonic implications

The basement of the North China Craton can be divided into the eastern, central and western zones, based on lithological, structural, metamorphic and geochronological data. The western zone comprises two different petrotectonic units: Archaean tonalitic–trondhjemitic–granodioritic (TTG) grey gneisses and metamorphic mafic rocks, and Palaeoproterozoic khondalite series. The former is characterized by isobaric cooling (IBC)-type anticlockwise P– T paths in the north-northwestern part of the zone and near-isothermal decompression (ITD)-type clockwise P– T paths in the eastern part, adjacent to the central zone. On the other hand, the tectonothermal evolution of Palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling, defining clockwise P– T paths. The Archaean TTG gneisses and associated mafic rocks with anticlockwise metamorphic P– T paths reflects an origin related to underplating and intrusion of mantle-derived magmas which may be derived from mantle plumes. They represent a late Archaean continental block in the western part of the North China Craton. The Palaeoproterozoic khondalite series rocks represent passive continental margin deposits. They were metamorphosed and deformed in the late Palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the North China Craton. The ITD-type clockwise P– T– t paths of the Palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the North China Craton at c. 1800 Ma.

Single zircon ages for granitoid gneisses in the Central Zone of the Limpopo Belt, Southern Africa and geodynamic significance

Ion microprobe, evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the Central Zone (CZ) of the Limpopo mobile belt, South Africa define three broad age groups, namely at 3188–3314, 2637–2734 and ca 2500 Ma, respectively. All these granitoids, including the youngest group, display polydeformational features suggesting that much of the tectonic history of the CZ occurred in the late Archaean and early Proterozoic. The oldest group of predominantly tonalitic–trondhjemitic–granodioritic gneisses was found in relatively small domains from the extreme W to the E of the CZ and probably represents remnants of a once more widespread early Archaean terrain. The most abundant granitoid gneisses belong to the 2.6–2.7 Ga age group and were emplaced into already ductilely deformed gneisses of the oldest group. Several of these rocks were derived from intracrustal melting. Rare metamorphic zircons with ages of ∼2560–2575 Ma reflect a high-grade metamorphic event in the CZ that has also been postulated by other workers. A strongly deformed orthogneiss with a protolith emplacement age of ∼2510 Ma proves beyond doubt that at least part of the structural history in the CZ occurred in the early Proterozoic. An anatectic granite in the extreme W of the CZ intruded at ∼2022 Ma and is probably related to the peak of granulite-facies metamorphism previously dated at 2027±6 Ma in the Messina area. The second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite-facies metamorphism in the Northern and Southern Marginal Zones of the Limpopo belt. Broad contemporaneity of extensive granitoid magmatism in all three Zones raises doubts about the exotic nature of the CZ and suggests a revised evolutionary model for the Limpopo belt.

A Rb–Sr isotopic study of the Archean rocks of the eastern Lac Seul region, English River subprovince, northwestern Ontario

Rb–Sr whole-rock data for the gneissic and granitic rocks of the eastern Lac Seul region, when combined with the U–Pb zircon dating of Krogh, document a history of multiple intrusion for the area. The oldest rocks are the Sen Bay plutonic complex gneisses which have complex Rb–Sr systematics. Interpretation of the Rb–Sr data yields model ages of 3000–3100 Ma which are in good agreement with a zircon age of 3040 Ma. The next oldest rocks are trondhjemitic–granodioritic gneisses with a Rb–Sr age of 2780 ± 90 Ma. The initial Sr ratio (I) of 0.7009 ± 4 for these rocks suggests that this age approximates the time of intrusion and that the magma was derived from lower crustal rocks with a very short residence lime in the crust. Following a period of deformation and metamorphism, granodioritic to granitic dikes, sills, and small plutons were intruded between 2660 and 2560 Ma ago. I values for these racks range from 0.7019–0.7027. If the I values of these rocks represent the source region for the granitic magmas, then one explanation for the I values would be that the magmas were derived from a source region of mixed lithology and age. The Sen Bay plutonic complex is considered to represent an earlier cycle of crustal formation which is distinct from a later 2800–2550 Ma old cycle which dominates much of the Superior Province.