Ga Granitic Rocks Research Articles

Mesoproterozoic (∼1.4 Ga) magmatism in the Liaoyuan Accretionary Belt, NE China: New implications for tectonic affinity and crustal evolution of microcontinents along the southern Central Asian Orogenic Belt

The Mesoproterozoic granitic magmatism has been identified in the middle and western part of the southern margin of Central Asian Orogenic Belt (CAOB), but rarely reported from the eastern part. In this study, Mesoproterozoic granitic rocks were identified in the Liaoyuan Accretionary Belt (LAB), an area located in the northeast China, within the eastern part of the southern CAOB. Information obtained from these samples is likely to elucidate the current understanding of the Precambrian evolution in this area. The five granitic rocks yielded zircon U–Pb weighted mean ages between 1321 ± 43 Ma and 1388 ± 31 Ma. These samples are characterized by high SiO2 (69.30–77.51 wt%), MgO (1.96–2.34 wt%), Fe2O3 (3.48–3.82 wt%), and Cr (40–80 ppm) contents, moderate Al2O3 contents (11.23–15.80 wt%), and high Mg# values (45–56). Furthermore, they are rich in light rare earth elements (LREEs), Zr, and Hf, but depleted in heavy rare earth elements (HREEs), Nb, Ta, and Ti. Moreover, they also exhibit significant negative Eu anomalies (Eu/Eu* = 0.25–0.84). In addition, the samples have high Zr + Ce + Y + Nb contents (300–497 ppm) indicating A-type granite affinities. The zircons depicted positive εHf(t) values (3.31 to 11.1, average value of 7.5) with two stage model ages (TDM2) varying from 1480 Ma to 2128 Ma. Based on the above data, it was concluded that the Mesoproterozoic granitic rocks in this area were derived from partial melting of juvenile crust with addition of mantle-derived materials. The granites were emplaced in an extensional setting caused by roll back or break off of subducted oceanic slab. Juvenile ∼ 1.4 Ga granitic rocks, like these in the LAB, were also preserved in other microcontinents along the southern CAOB. Their geochemical characteristics are comparable to rocks derived from contemporaneous igneous activity in Laurentia and Baltica, but not those in the North China, Tarim or the Siberian Cratons. Based on the present and previous studies, the microcontinents that preserved the juvenile ∼ 1.4 Ga rocks might belong to a continental terrane in the periphery of the Columbia supercontinent and show tectonic affinity with the combined Laurentia–Baltica landmass.

Petrogenesis and tectonic implications of the 2.1–2.0 Ga granitoids in Fuping Complex, North China Craton: Constraints from petrology, geochemistry and zircon U-Pb-Hf isotopes

Mid-Paleoproterozoic (2.2–2.0 Ga) magmatism in the Trans-North China Orogen (TNCO) is of great significance for the Paleoproterozoic geological evolution of the entire North China Craton (NCC). In this paper, we present geochemical, SHRIMP zircon U-Pb and Hf isotopic data on the 2.1–2.0 Ga granitoids in the Fuping Complex of the TNCO. These include the Nanying granitic gneiss, which yielded ages of 2075 ± 13 Ma, 2079 ± 9 Ma and 2073 ± 10 Ma, the Gangnan granitic gneiss, dated at 2081 ± 8 Ma, 2082 ± 14 Ma and 2063 ± 7 Ma, the Baiyangling granite with ages of 2077 ± 6 Ma, 2070 ± 19 Ma and 2069 ± 12 Ma, and the Shangzhuang granite, which yielded ages of 2072 ± 12 Ma and 2070 ± 12 Ma. The 2.1–2.0 Ga granitic rocks in Fuping Complex are enriched in SiO2, K2O + Na2O, rare earth elements (REEs) (except Eu), high field strength elements like Zr, Nb, Ga and Y, and depleted in CaO, Al2O3 and Sr. Their Ga/Al ratios and absolute Zr + Nb + Y + Ce contents are high and their average Zr saturation temperatures are as high as 890 °C, all of which are consistent with the features of A-type granite. The εHf(t) values of zircon in all samples are much lower than that of the depleted mantle at the same stage, while Hf model TDM1, TDMC ages have ranges of 2.3–2.7 Ga and 2.4–2.9 Ga, respectively, which are much older than the age of the granitoids. These zircon Hf model ages are similar to those of the Neoarchean TTG (Tonalite-Trondhjemite-Granodiorite) gneisses in the Fuping Complex. Therefore, based on a combination of petrology, geochemistry and isotopic characteristics, we infer that these four 2.1–2.0 Ga granitic plutons in the Fuping Complex are partial melts of the Neoarchean TTG gneiss generated in an extensional environment. Based on extensive data, the 2.1–2.0 Ga A-type granites and the mafic dikes in the Fuping Complex represent a bimodal magma association. Considering the sedimentary rocks of the Gantaohe Group, deposited in the same period, we deduce that the 2.1–2.0 Ga A-type granite in the TNCO Fuping Complex was emplaced in an intra-continental rift setting in the NCC. This evidence indicates that the North China Craton was initially stabilized at the end of the Neoarchean.

Neoarchean and Paleoproterozoic K-rich granites in the Phan Si Pan Complex, north Vietnam: Constraints on the early crustal evolution of the Yangtze Block

Precambrian igneous and metamorphic rocks of the Phan Si Pan Complex, North Vietnam, constitute the southern extension of the Yangtze Block, and provide a valuable record of the early evolution of the continental crust. We present results of U-Pb zircon geochronology and geochemistry for Precambrian granites in this complex to constrain their emplacement age and genesis. Granites from three plutonic bodies yielded ages of 2848 ± 15 Ma, 2768 ± 19 Ma and 1869 ± 30 Ma, which represent newly-recognized late Archean to Paleoproterozoic potassic granite plutonism in the southern Yangtze Block. The average εHf(t) values range from −6.2 to 0.1 for the 2.85 to 2.77 Ga granitic rocks and −13.1 to −9.2 for the ca. 1.86 Ga granitic rocks, with two-stage model ages of 3.64 to 3.20 Ga and 3.31 to 3.07 Ga, respectively, suggesting derivation from partial melting of Paleoarchean and Mesoarchean crust. The late Archean potassic granites exhibit high K2O, and high Sr/Y and (La/Yb)N ratios with negligible Eu anomalies, indicating derivation from melting of the thickened lower crust, which is inferred to have occurred in an active margin setting. The late Paleoproterozoic alkali feldspar granites are characterized by high FeOT/(FeOT + MgO)(0.96–0.99) and 10000 * Ga/Al (2.75–2.94) ratios, showing an affinity of A-type granite. These A-type granites exhibit flat chondrite-normalized HREE patterns and strong negative Eu anomalies, and low Sr/Y and (La/Yb)N ratios, corresponding to melting at a shallow depth, probably in a post-collisional extension setting.Comparison of the rock units and events recorded by the Phan Si Pan Complex with other Archean to Paleoproterozoic complexes (Houhe, Dongchuan, Yudongzi, Douling, Zhongxiang and Kongling complexes) in the Yangtze Block indicate spatially distinct histories of crustal growth, and thus may reflect independent terranes. The ca. 1.86 Ga post-collisional magmatism, which succeeds a 2.0–1.9 Ga metamorphic event, is distributed throughout the Yangtze Block, including the Phan Si Pan Complex, suggesting assembly of the disparate terranes and final cratonization of the Yangtze Block overlaps with, and may be related to, assembly of the Nuna supercontinent.

Early Paleoproterozoic magmatism in the Yangtze Block: Evidence from zircon U-Pb ages, Sr-Nd-Hf isotopes and geochemistry of ca. 2.3 Ga and 2.1 Ga granitic rocks in the Phan Si Pan Complex, north Vietnam

Our understanding of the early evolution of the Yangtze Block is limited by the sparsely dispersed nature of pre-Neoproterozoic exposures. New, integrated petrographic, zircon U-Pb age and Hf-Nd isotope analyses, and whole-rock geochemical data for early Paleoproterozoic granites in the Phan Si Pan Complex provides new insights into the evolution of the Yangtze Block as well as its role in the Pre-Nuna supercontinent. LA-ICP-MS zircon U-Pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207Pb/206Pb ages of 2306 ± 12 Ma and 2096 ± 15 Ma, respectively. Zircons from the quartz monzonite have εHf(t) values ranging from −4.1 to −2.1, corresponding to TDM2 model ages of 3002–2890 Ma, whereas zircons in the gneissic granite have εHf(t) values between −0.95 and +1.72 and corresponding TDM2 model ages of 2660–2516 Ma, which are consistent with their whole-rock Nd isotope values. Geochemically, the quartz monzonites are I-type granites. Combined with their relatively high Sr/Y ratios and low Y concentrations, as well as fractionated REE patterns with relatively high LREE but low HREE concentrations, they were probably generated by partial melting of the thickened middle-lower crust under elevated temperature. Geochemical and isotopic signatures suggest that the ca. 2.1 Ga gneissic granites are high-K calc-alkaline, ferroan A-type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material. The Phan Si Pan complex has a distinct early Paleoproterozoic crustal evolution history compared with the other crustal provinces of the Yangtze Block, suggesting independent histories that were not unified until the late Paleoproterozoic during the assembly of Nuna. Moreover, the magmatism and tectonic evolution of the north Vietnam region is broadly similar to that of the Arrowsmith Orogen of the Rae craton in Laurentia suggesting a potential spatial linkage. The geologic record of the Yangtze Block does not support an early Paleoproterozoic shutdown of plate tectonics.

Character and tectonic setting of plutonic rocks in the Gällivare area, northern Norrbotten, Sweden

ABSTRACTPetrographical and lithogeochemical investigations in combination with mapping in the Gällivare area, northern Norrbotten, Sweden, have led to the identification of several igneous intrusive rock types. These include: (1) ultramafic-mafic complexes, (2) mafic-intermediate rocks, (3) dolerites and (4) felsic plutons. The ultramafic-mafic rocks include the ca. 1.88 Ga Dundret complex and ca. 1.80 Ga Vassaravaara complex. The Dundret complex has tholeiitic to calc-alkaline affinity, shows a primitive mineral content and was formed in an extensional tectonic setting. The Vassaravaara complex has a similar chemical signature as the Dundret complex. The mafic-intermediate plutons vary in composition from gabbro to diorite. The chemical signature of the dioritic rocks indicate formation in a volcanic arc setting. Dolerites occur as solitary dikes and have calc-alkaline affinity. The felsic plutons include granite and syenite of ca. 1.88, 1.80 and 1.78 Ga age. The felsic plutons have calc-alkaline to shoshonitic affinity and mostly show a metaluminous I-type character. Results indicate subduction at 1.90 Ga resulting in a volcanic arc system, and including extensional events generating back-arc environments leading to mafic, intermediate and felsic magmatism in the Gällivare area. Subduction at 1.80 Ga is suggested to have caused a similar process generating mafic and felsic magmatic rocks in the same area. A subsequent collision event finally generated 1.78 Ga granitic rocks.

Early crustal evolution of the Yangtze Craton, South China: New constraints from zircon U-Pb-Hf isotopes and geochemistry of ca. 2.9–2.6 Ga granitic rocks in the Zhongxiang Complex

We report here new zircon U-Pb age and Hf-isotope as well as geochemical analyses of the recently discovered Archean-Paleoproterozoic Zhongxiang Complex in the northern-central Yangtze Craton, South China, and interpret the early crustal evolution of the Yangtze Craton. Zircon LA-ICP-MS dating yielded magmatic crystallization ages of ca. 2.90–2.87 Ga for two monzogranites, ca. 2.77 Ga for a trondhjemitic gneiss, and ca. 2.67–2.62 Ga for three potassic granites. The trondhjemitic gneiss also contains zircon rims that record a metamorphic event at ca. 2.08 Ga. These results demonstrate that the Zhongxiang Complex is significantly older than previously thought, and it contains the only potential TTG with ages of 2.8–2.7 Ga for the Yangtze Craton. These granitoids vary considerably in zircon Hf isotopic and geochemical characteristics. The ca. 2.90–2.87 Ga monzogranites correspond in composition to alkaline and calc-alkalic peraluminous granites originated from mixing of ancient and juvenile crustal components at relatively shallow depths. In contrast, the ca. 2.77 Ga trondhjemitic gneisses are calc-alkalic and peraluminous with relatively high Sr/Y and (La/Yb)N ratios and slightly evolved zircon Hf signatures, but without significant Eu anomaly that is, typical of a medium-pressure TTG component. Consequently, they are interpreted to be the products of partial melting of a basaltic source with appreciable involvement of pre-existing crustal components (i.e., tonalites), with garnet and amphibole in the residue. The ca. 2.67–2.62 Ga potassic granites are alkaline, faintly peraluminous and ferroan, exhibiting a high-silica A-type granite affinity. They are characterized by low Al2O3/TiO2 ratios with dominantly undepleted zircons, suggesting their derivation from melting of meta-sedimentary rocks under high-temperature conditions with plagioclase as the main residual phases. The Zhongxiang Complex thereby differs from the Kongling Complex and other Archean provinces in the Yangtze Craton by preserving distinct magmatic pulses (ca. 2.90–2.87 Ga; ca. 2.77 Ga; ca. 2.67–2.62 Ga) with different nature and sources. A comparative analysis of the crustal evolution of the Zhongxiang Complex and other Archean provinces of the Yangtze Craton indicates a compositionally heterogeneous Archean Yangtze Craton, which likely comprises several Archean terranes that accreted together to form a uniform craton during the late Archean to Paleoproterozoic time.

Geochemistry, U-Pb dating, and Lu-Hf isotopes of zircon and monazite of porphyritic granites within the Jiao-Liao-Ji orogenic belt: Implications for petrogenesis and tectonic setting

Abundant porphyritic granites, including Grt-bearing and Bt-bearing porphyritic granites, and flesh-red porphyritic granites, as various scale batholiths are widely distributed within the Jiao-Liao-Ji belt (JLJB), which extends for a distance of at least 1000km, rather than being limited to sporadic outcrops in southern Liaoning and Jilin as stated in previous studies. On the basis of morphology, CL and BSE zoning patterns, and mineral inclusions of dated zircon and monazite grains from samples of various porphyritic granites, the grains exhibit evident magmatic zones with distinct cores and rims and preserve felsic mineral inclusions of Qtz+Kfs+Pl+Qtz±Ap in both domains. The magmatic zircon cores and rims from 18 dated samples yielded two distinct weighted mean age groups, from 1872±9 to 1866±7Ma and 1851±14 to 1842±10Ma, with age peaks at 1869±1.7 and 1848±2.4Ma, respectively. Similarly, the magmatic cores and rims of monazites from 10 dated samples also showed two distinct weighted mean age groups, from 1870±3 to 1864±2Ma and 1851±6 to 1842±12Ma, with age peaks at 1868±0.8 and 1847±1.2Ma, respectively. The numerous U-Pb age data of both zircons and monazites revealed that the JLJB experienced a major episode of granitic magmatism in the Paleoproterozoic with initial emplacement timing of melts at ∼1.87Ga and crystallisation end timing of melts at ∼1.84Ga as represented by the magmatic cores and rims of the zircons and monazites, respectively. A combined study of whole-rock geochemistry and zircon Lu-Hf isotopic analyses revealed that the three types of porphyritic rocks are enriched in HREE, Zr, Nb, Ga, and Y and are depleted in V, Cr, and Ni, and that most have high TFeO/MgO ratios and Zr-saturation temperatures of ∼850°C, which is suggestive of an affinity with A-type granite. Moreover, the dominant εHf(t) values of the magmatic zircons range from −4.8 to +3.3, which is consistent with those of the 2.2–2.1Ga calc-alkaline meta-granites in the JLJB. Considering the high contents of garnet, Al-rich paragneiss enclaves, and ∼1.92Ga inherited metamorphic zircons in the granites, these porphyritic rocks were most likely derived from the partial melting of the 2.2–2.1Ga calc-alkaline meta-granites with abundant involvement of Al-rich pelitic granulites from the basement of the JLJB. As the result of a newly combined study of traditional geothermobarometers and phase equilibria modelling together with geochronological U-Pb dating, a precise clockwise P-T-t path with a subsequent near isothermal decompression process has been well determined in the HP-MP granulite facies metamorphic rocks in the JLJB. Thus, the following sequence of events can be inferred. Abundant ∼2.2–2.1Ga granitic rocks and voluminous Paleoproterozoic sedimentary rocks were subducted to lower crustal depths and underwent a peak HP-MP granulite facies metamorphism at 1950–1900Ma. These granulite facies rocks were exhumed to middle crustal levels and experienced a post-peak MP-LP granulite facies metamorphism at 1875–1840Ma. The retrograde rocks continued to be exhumed to shallow crustal levels at 1830–1800Ma and were finally overprinted by late amphibolite facies retrogression. The numerous magmatic zircons and monazites from the various porphyritic granites record a distinct Paleoproterozoic granitic magmatic event with two evident age peaks of ∼1.87Ga and ∼1.84Ga, which are completely consistent with the post-peak MP-LP granulite facies metamorphism (1875–1840Ma), representing an anorogenic magmatic association formed in an extensional setting possibly resulting from mantle upwelling and lithospheric thinning and resulting in the violent partial melting of ∼2.2–2.1Ga meta-granites along with abundant Al-rich pelitic granulites. Considering these data in combination with previous whole-rock geochemistry and age data, it can be stated that similar porphyritic granites are not only limited to the entire JLJB but are also widely distributed in North and South Korea. Thus, it can be concluded that the ∼1.87–1.84Ga porphyritic granites, together with massive rapakivi granites, Kfs-megacrysts, and porphyritic monzogranites, as products of an anorogenic granitic magmatic event, may have widely formed in both the JLJB and Korean Peninsula in an extensional and thinning tectonic setting.

Fluid compositions and P-T conditions of vein-type uranium mineralization in the Beaverlodge uranium district, northern Saskatchewan, Canada

The Beaverlodge district in northern Saskatchewan is known for “vein-type” uranium mineralization. Most of the uranium deposits are spatially related to major structures, and hosted by ca. 3.2–1.9Ga granitic rocks (and albitite derived from them) and by ca. 2.33Ga Murmac Bay Group amphibolite, all of which are unconformably overlain locally by deformed but unmetamorphosed redbeds of the ca. 1.82Ga Martin Group, and by the flat-lying ca. 1.75–1.5Ga Athabasca Group. The uranium mineralization is mainly hosted in fault rocks (breccias) and carbonate±quartz±albite veins, referred to as breccia-style and vein-style mineralization, respectively, with the latter being the focus of this study. Most of the mineralized veins occur in the basement rocks, although some crosscut the Martin Group. This study examines the field, petrographic, fluid inclusion and C-O isotope characteristics of mineralized and non-mineralized veins from 19 deposits/occurrences as well as from the Martin Group, with an aim to better understand the mineralizing environment and processes.The coexistence of liquid-dominated (L+V), vapour-dominated (V+L) and vapour-only (V) fluid inclusions within individual fluid inclusion assemblages (FIAs) in the veins suggests fluid immiscibility and heterogeneous trapping. The L+V inclusions with the lowest homogenization temperatures (Th) within individual FIAs are interpreted to represent homogeneous trapping of the liquid phase, which yield Th values from 78° to 330°C (mainly 100° to 250°C), and salinities from 0.2 to 30.8wt.% NaCl equivalent. Mass spectrometric analysis of bulk fluid inclusions shows that the volatiles are dominated by H2O (average 97.2mol%), with minor amounts of CO2, CH4, H2, O2, N2, Ar and He. Fluid pressures were estimated to be <200bars based on the inference of fluid immiscibility, fluid temperatures of 100° to 250°C, and low concentrations of non-aqueous volatiles (<3mol%). The δ18OVPDB and δ13CVPDB of carbonate minerals associated with mineralization range from −20.5 to −8.9‰ and −10.1 to −0.9‰, respectively. The δ18OVSMOW values of the parent fluids calculated using the Th values range from −9.6 to +17.0‰, with the majority from 0 to +5.0‰. O isotopes of paired equilibrium quartz and calcite, analyzed by secondary ion mass spectrometry (SIMS), yield temperatures from 161° to 248°C, which are consistent with the fluid inclusion data.The new fluid inclusion and stable isotope data are inconsistent with a metamorphic or magmatic-hydrothermal model as proposed in some previous studies (for breccia-style and vein-style mineralization), but rather support a model in which the vein-type uranium mineralization took place at relatively low temperature (100° to 250°C) and shallow (<2km) conditions, with fluid pressure fluctuating between hydrostatic and sub-hydrostatic regimes, possibly related to episodic faulting. The mineralizing fluids were mainly sourced from the Martin Lake Basin, and uraninite was precipitated as a result of mixing between this basin-derived fluid and fluids carrying reducing agents (Fe2+, CH4) derived from the basement, although fluid-rock reactions and fluid immiscibility may have also played a role.

Zircon U–Pb ages and Hf isotopic compositions indicate multiple sources for Grenvillian detrital zircon deposited in western Laurentia

Combined U–Pb ages and Hf isotopic data from 1.0 Ga to 1.3 Ga (Grenvillian) detrital zircon in Neoproterozoic and Cambrian siliciclastic sedimentary rocks in southwest North America, and from igneous zircon in potential Mesoproterozoic source rocks, are used to better assess the provenance of detrital zircon potentially transported across Laurentia in major river systems originating in the Grenville orogenic highlands. High-precision hafnium isotopic analyses of individual ∼1.1 Ga detrital zircon from Neoproterozoic siliciclastic sedimentary rocks in Sonora, northern Mexico, reveal that these zircons have low εHf(0) (−22 to −26) and were most likely derived from ∼1.1 Ga granitic rocks embedded in local Mojave Province Paleoproterozoic crust. In contrast, Grenvillian detrital zircons in Cambrian sedimentary rocks in Sonora, the Great Basin, and the Mojave Desert, have generally higher εHf(0) (−15 to −21) as demonstrated both by high precision solution-based, and by lower precision laser ablation, ICPMS data and were likely derived from more distal sources further to the east/southeast in Laurentia. Comparison to new and existing zircon U–Pb geochronology and Hf isotopic data from Grenvillian crystalline rocks from the Appalachian Mountains, central and west Texas, and from Paleoproterozoic terranes throughout southwest North America reveals that zircon in Cambrian sandstones need not entirely represent detritus transported across the continent from Grenville province rocks in the vicinity of the present-day southern Appalachian Mountains. Instead, these zircons could have been derived from more proximal, high εHf(0), ∼1.1 Ga, crystalline rocks such as those exposed today in the Llano Uplift in central Texas and in the Franklin Mountains of west Texas. Regardless of the exact source(s) of the Grenvillian detrital zircon, new and existing whole–rock Nd isotopic data from Neoproterozoic to Cambrian siliciclastic sedimentary rocks in the Mojave Desert demonstrate that the occurrences of higher εHf(0), Grenvillian detrital zircons are decoupled from the sources of the bulk of the sedimentary detritus in which the zircons are entrained. The Cambrian Wood Canyon Formation and the underlying “off craton” Neoproterozoic Johnnie Formation and Stirling Quartzite all contain higher εHf(0), Grenvillian detrital zircon, in some cases as the dominant detrital zircon population. However, only portions of the Wood Canyon Formation have whole rock Nd isotopic compositions consistent with a bulk sediment source in ∼1.1 Ga sources rocks. Whole rock Nd isotopic compositions of the remaining portions of this unit, and all of the Johnnie Formation and Stirling Quartzite, require bulk sediment sources principally in Paleoproterozoic continental crust. We consider the observed decoupling in the sources of Grenvillian detrital zircon and bulk sediment in the Wood Canyon Formation and underlying siliciclastic sediments as a demonstration that detrital zircon U–Pb and Hf isotopic data alone can provide an incomplete picture of the source of sediments that comprise a given siliciclastic stratigraphic unit.

Paleoproterozoic (ca. 2.1–2.0 Ga) arc magmatism in the Fuping Complex: Implications for the tectonic evolution of the Trans-North China Orogen

The Fuping Complex is exposed at the central domain of the Trans-North China Orogen (TNCO). Here we report new zircon LA-ICP-MS U–Pb ages, Lu–Hf isotope data, and whole rock geochemistry on a suite of meta-granitoids, amphibolites and metasedimentary rocks from the Fuping Complex. Magmatic zircons from the gneissic granite and syenogranites yield weighted mean 207Pb/206Pb ages of 2050±21Ma and 2077±16Ma, respectively. Those in two amphibolites show weighted mean 207Pb/206Pb ages of 2054±26Ma and 2044±61Ma. Zircons in a felsic schist define two age populations of 2450–2585Ga and 2004–2097Ga, suggesting the Fuping TTG gneisses and Nanying gneissic granites as the provenance. Our data also indicate that the Wanzi supracrustal sequence was deposited between 1.95 and 1.84Ga.The geochemical data suggest that the 2.1–2.0Ga granitic rocks and amphibolites from the Fuping Complex formed in subduction-related arc setting. The granitoids show SiO2 content between 72.53 and 77.67wt.% and classify as high-K shoshonitic, calc-alkalic to alkali granite. The amphibolites are subalkaline basalts in composition. Zircons from the 2.1–2.0Ga granitoids and amphibolites display positive ɛHf(t) values in the range of 0.3–8.5 and Hf crustal model ages (TDMC) from 2124 to 2639Ma. The 2.1–2.0Ga magmatic zircons in the Wanzi supracrustal assemblage show relatively lower ɛHf(t) values (−0.8 to −2.6) and Hf crustal model ages (TDMC) of 2710–2801Ma, suggesting that the magma sources of the 2.1–2.0Ga suite included reworked 2.7–2.8Ga crustal basement together with Neoarchean–Paleoproterozoic juvenile components. Based on the widespread 2.1–2.0Ga magmatism as recorded in different rock types within the TNCO, we propose a tectonic model where we envisage simultaneous subduction and continental rifting process between several micro-blocks (or complexes). During 2.1–2.0Ga, the Wutai Complex and the Fuping Complex were separated by the Longquanguan ocean. The double-sided subduction of the Longquanguan oceanic lithosphere resulted in distinct arc magmatism. Continental rift setting developed in the Hengshan, Huai’an and Zanhuang Complexes coevally. Finally, metamorphism associated with the ∼1.85Ga collision between the Western Block and the Eastern Block along the TNCO overprinted all the lithologies. The Paleoproterozoic continent building recorded in this study correlates with similar events in the crustal fragments of the Columbia supercontinent.

Ba-RICH K-FELDSPAR FROM MAFIC XENOLITHS WITHIN MESOARCHEAN GRANITIC ROCKS, BEARTOOTH MOUNTAINS, MONTANA, USA: INDICATORS FOR BARIUM METASOMATISM

A mafic xenolith included within 2.8 Ga granitic rocks from the eastern Beartooth Mountains of Montana, USA shows evidence of significant Ba metasomatism related to an influx of hydrous fluids under lower amphibolite-facies conditions, and is indicated by the development of “hyalophane”. The volumetrically dominant Mesoarchean host rocks include granodioritic-granitic-tonalitic gneisses that have variable and high barium (Ba) contents, with some rocks exceeding 3500 ppm Ba. The mafic xenolith retains some textural features inherited from its igneous tonalitic protolith, but it was affected by multi-stage metamorphic overprints. An initial amphibolite-facies overprint generated a distinctive texture, mineral assemblage, and mineral chemistry, but these features are locally replaced by mineral assemblages and chemistry that result from infiltration of lower-temperature Ba-rich hydrous fluids. The original amphibolite facies assemblage is magnesio-hornblende + plagioclase (An 35 ) + biotite + quartz + titanite + allanite + zircon. Thermobarometry indicates peak conditions of ~660 °C and ~7 kbar partially overprinted by a later, fluid-rich metamorphism at ~550 °C. Submillimeter-scale regions modified by the invasive fluid are characterized by a change in texture denoted by vermicular epidote, epidote overgrowths on allanite cores, myrmekitic plagioclase (~An 20 ), and complexly zoned hyalophane. Hyalophane grains locally have interior regions with a low celsian component (Cn,BaAl 2 Si 2 O 8 ) of ~Cn 7 flanked by more enriched zones of up to Cn 27 . Very fine exsolution lamellae of albite-hair perthite are observed in the central zone of the low-Cn “hyalophane”. The Ba-rich hydrous fluid is considered to have been derived from the surrounding granitic rocks. Ultimately, the most likely source of the Ba is subducted sediments in a Mesoarchean subduction zone where fluids generated from Ba-enriched sediments were incorporated into fluids associated with formation of the 2.8 Ga granitic rocks. Upon crystallization of the granitic rocks, a Ba-enriched metasomatic fluid was released to develop “hyalophane” in the mafic xenolith.