Felsic Research Articles

Consecutive underplating of cognate magmas: Contributions to the enclaves and large-volume plagiogranites in the North Qaidam orogen (NW China)

Magma mingling between mafic and felsic magmas is a plausible mechanism for the formation of mafic microgranular enclaves (MMEs) which are common worldwide. Through dissecting the MMEs with parentage of magma mingling, it can be ascertained how two types of magmas with contrasting physicochemical properties interplay and what implicit information magma mingling records in crust-mantle interaction. Here we present petrography, geochronology, major and trace elements, Sr-Nd-Hf isotopes, and mineral geochemistry for rock associations from the North Qaidam orogen, which provides new insights into the processes of magma mingling, petrogenesis of large-volume host plagiogranites and crust-mantle magmatic interaction. MORB (mid-ocean ridge basalt)-like and arc-like magmas can be distinguished from mantle-derived mafic samples with zircon UPb ages ranging from 480 to 465 Ma. MORB-like magmas originated from a MORB-type asthenospheric mantle, whereas arc-like magmas were derived from an ultra-depleted asthenospheric mantle as a result of consecutive extraction of MORB-like magmas and late-stage metasomatism by subduction-related aqueous fluids. Given the content discrepancies for major oxides, gabbro-dioritic MMEs are formed by incipient mechanical mingling between gabbro-diorite magmas and felsic plagiogranite magmas. MMEs and host plagiogranites yield coincident crystallization ages of 478 Ma and overlapped whole-rock SrNd ((87Sr/86Sr)i = 0.7044–0.7052; εNd(t) = +4.6 ∼ +5.6) and zircon Hf isotopes (εHf(t) = +13 ∼ +16). The host plagiogranite magmas are most likely derived by partial melting of pre-existing analogues of gabbro-diorite intrusions under the condition of consecutive underplating of different batches of cognate magmas over short time scales, which in turn determine the consistent isotope compositions between MMEs and host plagiogranites. As for mineral geochemistry, the plagioclases with the highest An% value and the hornblendes implying the highest melt water content, appear within the MME-bearing plagiogranites next to the chilled margin of MMEs, illustrating that aqueous fluid and heat transfer play a pioneering but finite role in incipient magma mingling. The consecutive underplating of cognate mantle-derived magmas as an ultimate engine leads to the partial melting of newly emplaced basites to form massive plagiogranite magmas and geochemically stratified crust. With magma ascent into a shallow level of crust, magma mingling between underplated mafic magmas and resultant plagiogranite magmas is finally realized.

Zircon U Pb geochronology and Sr-Nd-Pb-Hf isotope geochemistry of the Dahuabei pluton from the northern margin of North China Craton: Implications for petrogenesis, tectonic setting and Au Mo mineralization

The Late Paleozoic Dahuabei Pluton is located in the Wulashan area at the northern margin of the North China Craton (NCC), adjacent to the Hadamengou gold deposit, the largest gold deposit in Inner Mongolia, China. However, its petrogenesis and its relationship with AuMo mineralization remain debatable. In this contribution, a geochemical, isotopic, and geochronological study was carried out on the Dahuabei pluton to constrain its magma sources, petrogenesis, and tectonic setting, and further discuss the linkage between the magmatism and AuMo mineralization. The Dahuabei pluton mainly consists of monzogranite and syenogranite, with abundant mafic microgranular enclaves (MMEs) in the monzogranite. Zircon UPb dating shows that the ages of the Dahuabei pluton range from 357 to 353 Ma, suggesting an Early Carboniferous emplacement. The crystallization age of the MMEs (355.4 ± 3.2 Ma) is nearly simultaneous to that of host monzogranite (356.5 ± 2.0 Ma). Geochemically, the granitoids show high-K calc-alkaline and weakly peraluminous affinities and are classified as highly to moderately fractionated I-type granites. Moreover, they are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, resembling typical continental arc magmatic rocks. Isotopically, the granitoids have negative εNd(t) (−10.0 to −7.3) and εHf(t) values (−10.0 to −4.4), as well as old Nd (1923–1704 Ma) and Hf (1986–1697 Ma) model ages, suggest that the granitoids were mainly derived from partial melting of ancient mafic lower continental crust, with involvement of mantle-derived components. The sharp contacts, fine-grained igneous textures, numerous xenocrysts from host granites, and similar crystallization ages with host granites of the MMEs suggest a magma mingling process, in which the MMEs represent discrete mafic to hybrid magma globules formed by undercooling after the injection of hotter mafic magma into cooler felsic magma. Geochemical and isotopic data (εNd(t) of −6.2 to −5.4 and εHf(t) of −10.2 to −2.8) indicate that the parental magma of MMEs was derived from partial melting of the enriched subcontinental lithospheric mantle metasomatized by slab-derived fluids and had experienced significant fractional crystallization. The indistinctive chemical and isotopic contrast between MMEs and host monzogranite implies their chemical and isotopic exchange during various degrees of magma mingling. Integrating the geochronological, geochemical, and Pb isotope data of the Hadamengou AuMo deposit and the Dahuebei pluton, we propose that the magmatic-hydrothermal system emanated from the magma mingling event during the emplacement of the Dahuabei pluton might be favorable for the mineralization. The Late Devonian to Early Carboniferous magmatism may have been a trigger for the formation of the AuMo deposits in the northern margin of the NCC.

Crystallization conditions of plagioclase in volcanic complexes of Sulawesi island, Pacific Ocean

Mineralogical and thermobarogeochemical studies allowed us to reveal the crystallization conditions of plagioclase in volcanic complexes of Sulawesi Island (Pacific Ocean). The composition of plagioclases indicates the presence of three types of melts, which were involved in the formation of plagioclase phenocrysts in basalts, andesites and dacites, as well as crystallites in the groundmass of andesites and dacites. The analysis of glasses of heated melt inclusions in plagioclase indicates the involvement of two contrasting melts (mafic and felsic) in crystallization of phenocrysts. At the final stages of volcanic processes, plagioclase in matrix of andesites and dacites formed from another felsic melts with maximum alkali content. The presence of primary syngenetic melt and fluid inclusions in plagioclase phenocrysts indicates the phase separation of felsic magmas, which led to the crystallization of feldspar phenocrysts of different composition during the formation of dacites. Computational modeling in a program (Putirka, 2008) based on the composition of plagioclases, glasses of heated inclusions and glasses from the groundmass allowed us to estimate the crystallization temperatures of feldspar phenocrysts in basalts (1160–1120 ?С), andesites (930–890 ?С) and dacites (1020–950 ?С), as well as the feldspar microcrystals in the groundmass of andesites (875–810 ?С) and dacites (890–850 ?С). Keywords: crystallization conditions, plagioclase, melt inclusions, Sulawesi Island, Pacific Ocean.

The early Permian high-temperature felsic magmatism induced by slab breakoff in Southern Mongolia, Central Asian Orogenic Belt and its tectonic implications

The early Permian high-temperature felsic magmatism induced by slab breakoff in Southern Mongolia, Central Asian Orogenic Belt and its tectonic implications

Zircon SHRIMP U-Pb Ages, Geochemistry and Nd-Hf Isotopes of ∼1.0 Ga A-Type Felsic Rocks in the Southwestern Yangtze Block, South China: Petrogenesis and Tectonic Implications

Zircon SHRIMP U-Pb Ages, Geochemistry and Nd-Hf Isotopes of ∼1.0 Ga A-Type Felsic Rocks in the Southwestern Yangtze Block, South China: Petrogenesis and Tectonic Implications

The Gharuchah-Sofla intrusion (northwestern Sanandaj-Sirjan zone, Iran): Age, petrology, geochemistry and tectonomagmatic implications

The Cretaceous Gharuchah-Sofla intrusion is a composite batholith consisting of a variety of mafic to felsic rocks in the northwestern Sanandaj-Sirjan zone, Iran. Both its age and the relationship between various rock types in the intrusion are unknown. To address these issues, we conducted a detailed age, petrologic, and geochemical study of the intrusion, their country rocks, and mafic microgranular enclaves (MMEs) hosted therein. Plutonic rocks from the main mass of the intrusion, dated at ~110 Ma by in-situ zircon UPb method, are dominated by gabbroic, dioritic, and quartz dioritic with subordinate granodioritic rocks. Magmatic rocks occurring mostly along margins of the intrusion have Late Ediacaran zircon UPb ages (~560 Ma). Both the Early Cretaceous and the Ediacaran magmatic rocks are sub-alkaline; the former has I-type affinity whereas the latter has S-type affinity with a tendency towards I-type. Both sets of rocks are characterized by continental crust-like trace element patterns showing negative anomalies in Nb, Ta, Sr, P, Eu, and Ti of variable intensity. Early Cretaceous gabbroic rocks exhibit unfractionated rare-earth element patterns (La/Yb = 1.9–3.0) and negative ZrHf anomalies. In contrast, other rocks of the Early Cretaceous and Ediacaran magmatic rocks show fractionated rare-earth element patterns (La/Yb = 11.0–16.8) along with the lack of such anomalies. MMEs, which occur within Early Cretaceous plutonic rocks, have major and trace element compositions similar to the gabbroic rocks. Contrasting Nd isotopic ranges are present between the Early Cretaceous plutonic rocks and the MMEs (ɛNd = +2.0 to +4.1) and the Ediacaran magmatic rocks (ɛNd = −5.8 to −0.4). Based on the geochemical and isotopic data obtained, we suggest that (i) the Ediacaran magmatic rocks formed by melting of Cadomian basement rocks below the blocking temperature of UPb exchange between zircon and melt, (ii) the MMEs represent frozen melts (i.e., calc-alkaline basalts) that formed the gabbroic rocks in the Early Cretaceous instruction, and (iii) fractional crystallization with slightly concomitant assimilation of Cadomian basement rocks was a key process forming intermediate and silicic rocks of the intrusion. The ~110 Ma Gharuchah-Sofla intrusion was likely associated with a continental margin arc related to the subduction of the Neotethys, providing solid evidence that this segment of the arc was built directly on the Cadomian basement.

Control of micro-wettability of pore-throat on shale oil occurrence: A case study of laminated shale of Permian Lucaogou Formation in Jimusar Sag, Junggar Basin, NW China

The control of micro-wettability of pore-throat on shale oil occurrence in different types of reservoir spaces remains unclear. Take the shale oil reservoir of the Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin as an example, the reservoir space in laminated shale and the control of micro-wettability of pore-throat on shale oil occurrence were studied by using scanning electron microscope (SEM), multi-stage pyrolysis, quantitative fluorescence, nuclear magnetic resonance (NMR) and other techniques. The results show that there are mainly two types of laminated shale in the Lucaogou Formation, namely laminated shale rich in volcanic materials + terrigenous felsic, and laminated shale rich in volcanic materials + carbonate. The former type contains feldspar dissolution pores and intergranular pores, mainly with felsic mineral components around the pore-throats, which are water-wet and control the free shale oil. The latter type contains carbonate intercrystalline pores and organic pores, mainly with oil-wet mineral components around the pore-throats, which control the adsorbed shale oil. The oil-wet mineral components around the pore-throats are conducive to oil accumulation, but reduce the proportion of free oil. In the Lucaogou Formation, free oil, with high maturity and light quality, mainly occurs in the laminated shale rich in volcanic materials + terrigenous felsic.

Post-Subduction Granite Magmatism and Gold-Sulfide Mineralization in the Abu Zawal (Fatira) Area, Eastern Desert, Egypt

Gold-sulfide mineralization in the Abu Zawal (Fatira) mine area, North Eastern Desert of Egypt, is related to porphyritic felsite dikes and elongate silicification zones in granitic rocks. These felsite dikes and the host granitic rocks exhibit major and trace element geochemical features typical of calc-alkaline and metaluminous I-type granites, likely originated in a late-orogenic setting. Their geochemical characteristics along with their fractionated LREE relative to HREE patterns imply either formation in a subduction-related environment or generation from subduction-modified source materials. Partial melting of subduction-metasomatized lower crustal rocks during extension following the lithospheric thickening may account for the production of such fertile, high Sr/Y and La/Yb magmas. In the Abu Zawal (Fatira) area, NE-trending altered felsite dikes, hydrothermal breccias, quartz enclaves, and wall-rock replacements are characterized by disseminations of chalcopyrite, pyrite, and rare gold. Alteration mineralogy, dominated by sericite, drusy quartz, kaolinite, calcite, and specular hematite, combined with the available fluid inclusion data suggests moderate to low temperature and near neutral pH conditions. The geochemical data of the altered wallrocks and mass balance calculations indicate significant mass losses in the altered rocks consistent with fluid/wallrock ratios higher than unity and near neutral pH conditions. Considering that the silica-rich host rocks, hydrothermal alteration, and sulfide-bearing hydrothermal quartz breccia in Fatira mine area were intuitively related to sulfur-saturated, oxidized felsic magmatism and associated hydrothermal systems, they are most likely linked to the post-subduction felsite porphyries (post-Hammamat felsites ~ 607 Ma), or pertaining to the late phases of the subaerial high-K calc-alkaline volcanics (Dokhan Volcanics ≤ 620 Ma).

Geochemical characteristics of the Middle–Late Permian sedimentary rocks in the southern Great Xing'an Range, NE China, and their constraints on the closure time of the Paleo Asian Ocean (Eastern Segment)

The Late Paleozoic sedimentary environment in the southern Great Xing'an Range in Northeast (NE) China, and the timing of the final closure of the Paleo Asian Ocean between the North China Craton and combined NE China blocks have been controversial for a long time, affecting and restricting the exploration and development of new area and new strata in the North Area in China. In order to reconstruct the structural-sedimentary evolution history of the southern Great Xing'an Range during the Middle to Late Permian, and to explore the closure time of the Paleo Asian Ocean (Eastern Segment), in this paper, the sedimentary characteristics and tectonic background of the Middle–Late Permian sedimentary rocks from well MKD1 are systematically analyzed by using elemental and organic geochemistry methods. The results show that the Middle Permian sedimentary records in well MKD1 show typical marine sediments, while the Late Permian sediments change significantly to clastic rocks deposited in the marine–continental transitional environment. The provenances of the Middle Permian rocks are mainly intermediate and felsic igneous rocks from continental active margin and oceanic island arc tectonic setting, related to the northern accretion complex, and the Songnen Block and partially arc magmatic rock. The source materials of the Late Permian rocks are mainly felsic rocks and some intermediate igneous rocks from the continental arc and active margin, related to the northern accretion complex, the arc magmatic rock and the Songnen Block. Depositional discontinuity after the Late Permian, regional lackness of the Early Triassic strata, sedimentary environment variation, tectonic setting and provenance location discrimination of the Middle–Late Permian rocks comprehensively reveal that the Paleo Asian Ocean (Eastern Segment) possibly closed in the late Late Permian. Finally, we propose a structural-sedimentary evolution pattern of the southern Great Xing'an Range during the Middle to Late Permian.

Main stages of the Kostomuksha greenstone belt Banded Iron Formation genesis, Karelian Craton: based on U-Th-Pb dating of zircon

We studied zircon from banded iron formations (BIFs) in the Kostomuksha greenstone belt (KGSB), Karelian Craton, for the first time. Here, BIFs are associated with Mesoarchean basalt-komatiites – BIF-1 and felsic volcanics – BIF-2, as well as with Neoarchean graywackes and felsic volcanics – BIF-3. All the BIFs contain accessory zircon. Analysis of its isotopic age shows the presence of 1-4 age clusters in each sample. There are single grains of detrital zircon in most of the studied BIFs. The grains are of commensurable or older age than the BIF enclosing rocks. The ages of detrital zircons differ among BIF association: 2.84 Ga in BIF-1, no detrital zincon in BIF-2, and 2.98–2.75 Ga in BIF-3. Most of the zircon in the BIFs is of metamorphic origin. All BIF groups contain 2.75–2.72 Ga zircon, while BIF-3 also has 2.65–2.64 Ga zircon associated with felsic magmatism. Rims and grains of 1.89–1.85 Ga zircon from BIF-1 and BIF-3, indicative of local tectono-thermal processes simultaneous with the formation of the Svecofennian Orogen, are reported here for the first time.

Spatial–temporal influence of the Mongol–Okhotsk and Palaeo–Pacific tectonic systems in NE China: Evidence from geochronological and geochemical data of felsic rocks in the east‐Central Jilin Province and southern Great Xing'an Range

The temporal and spatial extents of the Palaeo–Pacific and Mongol–Okhotsk tectonic systems, which together dominated the Mesozoic tectonics of NE China, still remain unclear. Here, we present new zircon U–Pb ages, isotopic analyses and whole‐rock major‐ and trace‐element data for felsic rocks from west and east Songliao Basin to explore the above question. The granites in Shancheng area formed between 176 and 164 Ma and have εHf(t) values of −22.09 to −7.59 with a TDM2 age range of 3622–2336 Ma. The granites are dominated by adakites and I‐type granite whose primary magma is derived from a thickened ancient crust. The rhyolites in Wushijiazi area formed between 151 and 141 Ma and have εHf(t) values of 3.34–12.38 with a TDM2 age range of 517–1328 Ma. They are A‐type granite and have a flat rare‐earth‐element (REE) pattern with obvious negative Eu anomalies, suggesting they were derived from partial melting of lower crustal rocks. The above findings indicate that the middle–late Jurassic granite in the Shancheng area formed in a compression setting related to the subduction of the Palaeo–Pacific Plate beneath the Eurasian continent and the Late Jurassic to early Cretaceous rhyolites in Wushijiazi area formed in an extensional setting related to the collapse of the Mongol–Okhotsk belt. In combination with regional geologic data, we conclude that the Palaeo–Pacific tectonic system mainly affected the east side of Songliao Basin, while the Mongol Okhotsk tectonic system mainly affected the Erguna Block, through the GXAR to the northern margin of the North China Craton (NCC).

The evolution of Kerguelen mantle plume and breakup of eastern Gondwana: New insights from multistage Cretaceous magmatism in the Tethyan Himalaya

The relationship between the breakup of eastern Gondwana and Kerguelen mantle plume remains extremely controversial. The crucial conundrum is that when and where the Kerguelen mantle plume initially formed and its evolutionary history are unclear. In this paper, we report the oldest Cretaceous bimodal magmatism (144–140 Ma) associated with the Kerguelen mantle plume from the Comei large igneous province in the Tethyan Himalaya. The mafic rocks are dominated by oceanic island basalt (OIB)-like dykes with occasional normal mid-ocean ridge basalt (N-MORB)-like intrusions, while the felsic rocks show A-type granites affinities. Our results suggest that the > 140 Ma bimodal magmatism was clearly a response to early Kerguelen plume activity, and the Kerguelen plume initially formed underneath the Tethyan Himalaya but not the triple junction between Australia, Antarctica, and Greater India. We argue that the Cretaceous magmatism associated with the Kerguelen plume in the Tethyan Himalaya can be divided into three pulses of ca.141 Ma, ca.132 Ma, and ca.117 Ma, which correspond to vertical underplating, continuous ascent, and a southward migration process, respectively. Moreover, we propose that the multistage OIB-like magmas in the Tethyan Himalaya are mainly the products of interactions between the lower crust and mantle plume, while the contribution of the lithospheric mantle is negligible. This proposal implies that the lithosphere was significantly thinned before ca.141 Ma, and the Kerguelen plume had been incubating underneath the Tethyan Himalaya prior to this. Our findings suggest that the Kerguelen plume may not have been the trigger for the initial breakup of eastern Gondwana, but was complementary to the later complete breakup process. We also innovatively consider that the northward dragging and slab-pull forces caused by the Cretaceous intra-oceanic subduction of the Neo-Tethys Ocean may have played an active role in the evolution of the Kerguelen plume and the breakup of eastern Gondwana.

Dominance of felsic continental crust on Earth after 3 billion years ago is recorded by vanadium isotopes

The transition from mafic to felsic upper continental crust (UCC) is crucial to habitability of Earth, and may be related to the onset of plate tectonics. Thus, defining when this crustal transition occurred has great significance for the evolution of Earth and its inhabitants. We demonstrate that V isotope ratios (reported as δ51V) provide insights into this transition because they correlate positively with SiO2 and negatively with MgO contents during igneous differentiation in both subduction zones and intraplate settings. Because δ51V is not affected by chemical weathering and fluid-rock interactions, δ51V of the fine-grained matrix of Archean to Paleozoic (3 to 0.3 Ga) glacial diamictite composites, which sample the UCC at the time of glaciation, reflect the chemical composition of the UCC through time. The δ51V values of glacial diamictites systematically increase with time, indicating a dominantly mafic UCC at ~3 Ga; the UCC was dominated by felsic rocks only after 3 Ga, coinciding with widespread continental emergence and many independent estimates for the onset of plate tectonics.

Pedochemical behaviors of rare earth elements in soil profiles along a lithosequence in eastern Taiwan

The composition and concentration of rare earth elements (REEs) in soil are primarily dependent on the lithology of parent materials. The different association preferences among individual REE and soil solid phases also cause REEs fractionation during pedogenetic processes. However, studies on the interaction between lithology and pedogenesis, affecting the geochemical behaviors of REEs, are lacking. Thus, this study elucidated the influence of parent materials on REEs concentration and distribution during pedogenesis in four pedons along a lithosequence covering schist, andesite, shale, and mafic rocks identified in eastern Taiwan. The highest ΣREEs was observed in the schist pedon (253–319 mg kg−1), followed by the andesite pedon (239–277 mg kg−1), mafic pedon (243–254 mg kg−1), and shale pedon (195–222 mg kg−1). Moreover, REE patterns and ratios of light REEs (LREEs) to heavy REEs (HREEs) indicated that soils derived from felsic rocks (schist and shale) tended to be rich in LREEs; conversely, soils derived from mafic rocks were rich in HREEs. HREEs decreased with an increase of Si, but values of REEs fractionation proxies (LaN/YbN and LaN/SmN) increased with increasing Si. The mass transfer coefficients (τ) of REEs, LREEs, and HREEs exhibited a significant correlation with LaN/SmN. Compared with LREEs, HREEs exhibited a greater depletion and contribution in REEs fractionation in the studied pedons, as evaluated based on τHREEs, w and its correlation with LaN/SmN and GdN/YbN. The relatively high mobility of HREEs, coupled with the clear illuviation, resulted in decreased values of LaN/SmN and GdN/YbN along with the increasing degree of soil weathering. This study elucidated the shift in the factors controlling the REEs fractionation from the lithologic characteristics of parent materials to the pedogenesis, along with the progressed soil development. Additionally, the increasing degree of soil weathering can be quantified using the ratios of LaN/SmN and GdN/YbN.

Quantifying Continental Crust Thickness Using the Machine Learning Method

AbstractCrustal thickness plays a key role in many geological processes. However, it remains challenging to quantify crustal thickness in the geological past. Here we propose an Extremely Randomized Trees algorithm‐based machine learning model to recover crustal thickness of old geological regions. The model is trained using major oxide and trace element compositions of 1,480 young intermediate to felsic rocks from global arcs and collisional orogens and geophysical measurements of crustal thickness. The model provides better estimations of crustal thickness than the commonly used methods based on Sr/Y and (La/Yb)N when applied to the testing data. The validity of this model is further demonstrated by its applications to the Kohistan–Ladakh, Gangdese and Talkeetna arcs, where paleocrustal thicknesses have been well constrained. We then use this model to construct the Mesozoic crustal thickness evolution of the Erguna Block in the southeast of the Mongol–Okhotsk suture belt. The closure time of the suture zone is still debated. Our results suggest that the crustal thickness of the Erguna Block increased from 43 ± 9 km at 210 Ma to 62 ± 7 km at 180 Ma, remained constant between 180 and 150 Ma, and then thinned to 36 ± 4 km at 120 Ma. These results suggest that the Mongol–Okhotsk Ocean closed in the Early–Middle Jurassic and the thickened crust was stretched during the Cretaceous. We show that the thick crust and compression‐extension transition seem to be favorable for the formation of porphyry copper deposits in the Erguna Block during the Late Jurassic.

Detrital zircon U–Pb isotopes and whole‐rock geochemistry of early Palaeozoic sediments of the Baoshan and Lancang Blocks, SW China: Implications for Proto‐Tethys evolution and Gondwana reconstruction

The palaeogeographic reconstruction of the microcontinents on the northern margin of Gondwanaland and the tectonic evolution model of the Proto‐Tethys in the Early Palaeozoic remain controversial. The Baoshan and Lancang Blocks in SW Yunnan, China, were located on the northern margin of the Gondwana continent during the Early Palaeozoic. In this study, we present new zircon U–Pb ages and whole‐rock geochemistry data for the sedimentary rocks of the Lancang Group in the Lancang Block and the Mengtong Group in the Baoshan Block to reveal the nature of sedimentary provenance with a further tectonic evolution model of the Proto‐Tethys. Our new detrital zircon U–Pb results, in conjunction with the published age of the metamorphic volcanic rock interlayers, suggest that the Mengtong and Lancang groups were probably deposited during the Early Palaeozoic. The Mengtong and Lancang groups have similar detrital zircon age spectra and display three distinctive age peaks at 1.2–1.1 Ga, ~0.95 Ga, and 0.6–0.5 Ga. This implies that the Baoshan‐Lancang Block was situated in the transitional zone between India and Australia on the Gondwana margin and received detritus from both cratons. Combined with abundant volcanic rocks in the Lancang Group and SSZ‐type ophiolites in the Changning‐Menglian belt, the Lancang Block should be reconstructed as a continental island arc of the Baoshan Block in the Early Palaeozoic. The geochemical analysis results show that the sediments of the two strata were primarily sourced from felsic rocks in the upper crust, and they were deposited in the active continental margin. Combined with previous research, we propose a new tectonic evolution model for the Proto‐Tethys Ocean in the Early Palaeozoic, from bisubduction to collisional orogeny.

Geochronology of the Daitari Greenstone Belt, Singhbhum Craton, India

The Singhbhum Craton in India contains well-preserved Palaeoarchaean greenstone belts that rival their Australian and South African counterparts, but their stratigraphic make-up remains to be resolved. Here we provide new SHRIMP- and LA-ICP-MS-based U–Pb and Lu-Hf data on magmatic and detrital zircons from stratigraphically resolved samples of the Daitari Greenstone Belt (DGB) in the south of the Singhbhum Craton. A minimum age for the Kalisagar Formation, the lowermost mafic–ultramafic volcanic suite of the DGB, is provided by a felsic sill dated at 3505 ± 6 Ma. Felsic volcaniclastic rocks from the overlying Talpada Formation provided a weighted mean age of 3507 ± 5 Ma. Zircon εHf(t) indicate a juvenile source for 3.5 Ga felsic volcanic rocks. Erosion of the volcanic edifice of the Talpada Formation acted as a predominant source for the overlying turbidite sequence of the Sindurimundi Formation, having identical U-Pb detrital zircon ages of ∼ 3505 Ma. A maximum age of deposition of 3502 ± 2 Ma is proposed for the Sindurimundi Formation. Magmatic zircon ages obtained from different intrusive felsic rocks in the Daitari belt ranges from c. 3.37 to 3.35 Ga and support the notion of widespread felsic magmatism of the Singhbhum Suite in the southern part of the craton. Xenocrystic zircon crystals dated at c. 3.55 to 3.52 Ga may correspond to an earlier phase of magmatism. 3.5 Ga marks an important period in the Earth’s history, when extensive felsic magmatism facilitated continental crustal growth as recorded in the Singhbhum, Kaapvaal and Pilbara cratons.

On the use of laser-induced breakdown spectroscopy data for mineralogical investigations – constraints and application of a clustering method

This study examines how laser-induced breakdown spectroscopy (LIBS) data collected using a downhole deployable LIBS prototype for geochemical analysis in a fashion that imitates downhole deployment may be used for mineralogical investigations. Two chemically and mineralogically practically identical felsic rocks, namely granite and microgranite, are used to assess the effects of rock texture on mineral classification and high-resolution SEM-TIMA (scanning electron microscope coupled with TESCAN's integrated mineral analyser) mineral maps are used to reveal mineralogical composition of each LIBS ablation crater. Additionally, in order to extend the LIBS application for fast mineralogical studies to a greenfield scenario (i.e. no previous knowledge) a clustering methodology is presented for mineralogical classification from LIBS data. Results indicate that most LIBS spot analyses sample mineral mixtures, 91.2 and 100% for granite and microgranite, respectively, which challenges mineralogical classification, particularly for fine-grained rocks. Positive identification and classification of minerals of slightly different compositions relative to the bulk rock (i.e. fluorite and biotite in granitic rocks) demonstrates how minerals or minerals groups of distinct and interesting chemical compositions (e.g. sulfides or oxides in silicate-dominated rocks) can be rapidly recognized in a mineral exploration scenario. Strategies for overcoming mineral mixture issues are presented and recommendations are given for effective workflows for mineralogical analysis using LIBS data in different mineral exploration stages. Supplementary material : Complete LIBS analyses for granite and microgranite samples (Appendix A); modal mineralogy results for each LIBS ablation crater based on TIMA analysis (Appendix B); and TIMA mineral maps for granite and microgranite samples are available at https://doi.org/10.6084/m9.figshare.c.6444482

Geochemistry and zircon geochronology of the Late Triassic volcanic–sedimentary successions in northern Tibet: Implications for the provenance and tectonic evolution of the Mesozoic Qiangtang Basin

The Late Triassic volcanic–sedimentary successions deposited in the Qiangtang Basin are linked with the initiation of the Mesozoic Qiangtang Basin. However, the nature of this basin is hotly debated. Here, we synthesize petrographic, mineralogical, geochemical, and detrital zircon U–Pb age data of these volcanic–sedimentary successions to constrain their chemical weathering history, provenance, and tectonic setting and to reconstruct the tectonic evolution of the Mesozoic Qiangtang Basin. The chemical weathering indices, especially the chemical index of alteration (CIA), are controlled by grain size and hydraulic sorting rather than sediment recycling. Low compositional and textural maturities, high index of compositional variation (ICV) values, and low CIA and plagioclase index of alteration (PIA) values in most samples suggest that the sediments from the Fanghu and north Woruoshan sections are predominantly from low-maturity sources that may have experienced a weak chemical weathering history. Based on provenance-sensitive geochemical indices (La/Th versus Hf, La/Sc versus Co/Th, and Sc/Th versus Cr/Th) and petrographic features, the detrital contributions to the Nadi Kangri Formation siliciclastic rocks are predominantly proximal felsic rocks. Detrital zircon age spectra suggest that the Nadi Kangri Formation siliciclastic rocks exhibit a provenance change from the Fanghu to the well QK-1 area, although all of the rocks were derived from the Qiangtang terrane itself. The Fanghu section was primarily sourced from synchronous magmatic rocks and pre-Triassic strata from the North Qiangtang and Central uplift belt, while well QK-1 was primarily sourced from recycled sediments from the South Qiangtang and Central uplift belt. Therefore, we propose that the Nadi Kangri Formation volcanic–sedimentary successions were deposited in a rift basin and that its evolution was likely related to the interaction of the rapid expansion of the Meso-Tethys Oceanic crust, collapse of the collisional orogen, and rollback of the Jinsha Paleo-Tethys subducting slab during the Late Triassic.

The Eoarchean Muzidian gneiss complex: Long-lived Hadean crustal components in the building of Archean continents

The nature of Earth's first crust and the processes that formed it are poorly constrained due to limited exposures of >3.7 billion-year-old (Ga) rocks. Here we report the discovery of a new Eoarchean terrane named the Muzidian gneiss complex in the Yangtze craton of central China where rocks as old as 3.81 Ga are preserved. In this study, we characterized six samples (including five TTGs and an amphibolite) through zircon U-Pb age, Hf isotope and bulk-rock 146,147Sm-142,143Nd isotope compositions. The subchondritic zircon initial Hf isotope compositions and the negative μ142Nd values for the 3.81 – 3.65 Ga samples reveal that these rocks were most likely reworked from >4.3 Ga mafic crust. The 2.5 – 2.4 Ga rocks in the same complex are the youngest felsic rocks on Earth with deficits in 142Nd, highlighting the long-lived role of Hadean crustal components in the building of a stable continent. The 147Sm-143Nd isotopic systematics of these rocks are disturbed. The currently available data for global Eoarchean rocks suggest two distinct lineages for > 3.6 Ga Eoarchean crustal blocks, one produced by reworking of Hadean mafic crust with the isotopic signature of coupled negative εHf(t) and μ142Nd, and the other by melting of incompatible-element-depleted mantle sources residual to Hadean crust formation characterized by positive μ142Nd and near chondritic εHf(t). The Eoarchean samples of the Muzidian gneiss complex in the present study, along with >3.6 Ga rocks in the Acasta, Napier and Nuvvuagittuq regions imply a Hadean crustal source distinct from the products expected for magma ocean crystallization.