Metabasaltic Rocks Research Articles

Synchronous late Neoarchean Na- and K-rich granitoid magmatism at an active continental margin in the Eastern Liaoning Province of North China Craton

The appearance of voluminous K-rich granitoid rocks during Late Archean marks gradual maturation and stabilization of the continental crust. While most K-rich granitoid magmatism followed TTG magmatism, they occur synchronously sometimes, and this relationship is crucial for our understanding of the evolution of late Archean continental crust and its geodynamic setting.In this study, a series of ~2.57–2.52 Ga coeval and diverse granitoid rocks, including quartz dioritic-trondhjemitic and granodioritic to monzo-/syenogranitic gneisses, were identified in the southern Fushun area of Eastern Liaoning Province, North China Craton. The ~2.57 Ga quartz dioritic gneisses show moderate MgO (≤ 3.96 wt.%) and moderate to high Mg# (37.3–75.5). Geochemical modeling, together with mildly fractionated REE patterns and negative Eu anomalies and depleted zircon ƐHf(t2) (+2.1 − +7.4), suggest that they were differentiated from a depleted mantle source that was metasomatized by slab-derived fluids. The younger trondhjemitic gneisses (~2.55–2.52 Ga) are divided into two subgroups, i.e., an older subgroup (~2547–2540 Ma) characterized by mildly fractionated REE patterns and negative Eu anomalies, and a younger subgroup (~2533–2517 Ma) with strongly fractionated REE patterns and positive Eu anomalies. The trondhjemitic gneisses lack evidence for magma differentiation, and they are inferred to have been formed by reworking of amphibolites/greywackes at diverse crustal levels, with some inputs of mantle materials in the earlier subgroup. The coeval ~2550–2529 Ma granodioritic and monzo-/syenogranitic gneisses are characterized by high K2O/Na2O (0.67–2.45) but low MgO and Mg# (mostly <2 wt.% and < 50, respectively). Geochemical modeling data indicate that the granodioritic gneisses are less differentiated, which could have been derived from the partial melting of high-K mafic rocks (e.g., regional late Neoarchean calc-alkaline meta-basaltic rocks), as further supported by the constant A/CNK (1.00–1.14) and zircon ƐHf(t2) (+2.0 − +9.3) values. In comparison, the monzo-/syenogranitic gneisses show variable A/CNK (0.78–1.32) and zircon ƐHf(t2) (−2.4 − +7.8) values. They are explained to be formed by the partial melting of mixed sources of high-K mafic and sedimentary rocks, with the primary magmas showing plagioclase and apatite fractionation.Taken together, the above granitoid rocks record gradually decreasing zircon ƐHf(t2) values and increasing crustal thickness. Considering the petrogenetic information, regional geological data and the presence of some ~3.45–2.70 Ga crustal materials, the late Neoarchean granitoid magmatism of southern Fushun were likely generated via complex crust-mantle interaction processes at an active continental margin. It is further emphasized that Archean active continental margins are key sites for the initial maturation of the crust, and this tectonic scenario acted as a trigger for granitoid diversification during both the subduction and subsequent collisional stages.

Late Cretaceous-Palaeocene subduction-collision-exhumation of a microcontinent along the northern, active margin of the Southern Neotethys: Evidence from the Alanya Massif and the adjacent Antalya Complex (S Turkey)

Abstract The Alanya Massif represents a microcontinent that underwent subduction/exhumation related to collision with the larger Tauride microcontinent to the north during latest Cretaceous-Palaeocene. Intermediate-structural-level blueschist melange in the northwest (Gundogmus area) encompasses ductile-deformed meta-basaltic rocks, meta-cherts and minor serpentinite in a lower-grade meta-mudrock matrix. The meta-basaltic protoliths mainly formed in a subduction-influenced setting, together with less common OIB and E-MORB. Late Permian platform/slope carbonates were intersliced during late-stage brittle thrusting and directly over-ridden by a lower-grade (greenschist facies) Late Permian-Early Triassic-aged platform unit. The blueschist melange is interpreted as an oceanic subduction complex that was obducted southwards related to microcontinental collision. Intermediate-structural-level HP/LT blueschist-ecologites farther south (Alanya area) indicate that the Alanya Massif represents variably subducted/exhumed continental crust. Both subduction and exhumation took place during the latest Cretaceous, prior to Late Palaeocene-Early Eocene marine transgression. Late Precambrian protoliths of eclogitic HP/LT rocks, and also similar-aged amphibolite facies meta-basic intrusions farther southeast (Anamur area) are interpreted to have formed in a Panafrican (Cadomian) late-stage back-arc setting. Late Triassic basaltic rocks in the Antalya Complex (structurally beneath and to the north) erupted in a rift setting. We restore the Mesozoic palaeogeography from north to south as: Tauride microcontinent-Antalya rift basin-Alanya micrcontinent-S Neotethys. The subduction/exhumation history may serve as a model for similar active margin/collisional settings elsewhere.

Temperature-dependent Rutile Solubility in Garnet and Clinopyroxene from Mantle Eclogite: Implications for Continental Crust Formation and V-based Oxybarometry

Abstract Despite its accessory mineral status in metabasaltic rocks, rutile controls the whole-rock Ti, Nb and Ta budget. These are key elements used to trace fluid- and melt-mediated mass transfer across the mantle–crust boundary. Rutile also contains significant amounts of the redox-sensitive element V, which is increasingly used to estimate oxygen fugacity. Kimberlite-borne mantle eclogite xenoliths, which are frequently rutile-bearing, have been interpreted as residues from the extraction of silicic partial melt similar in composition to the average continental crust. Published mineral compositions for eclogite xenoliths from various cratons combined with geothermobarometrical calculations show that TiO2 contents in garnet and clinopyroxene increase with increasing temperature of last residence in the lithospheric mantle, whereas apparent clinopyroxene–garnet distribution coefficients decrease. This implies that (1) increasing TiO2 contents in eclogitic garnet or clinopyroxene are not a signature of increasing metasomatism with depth, (2) whole-rock eclogites reconstructed without rutile will increasingly underestimate TiO2, Nb and Ta contents with decreasing temperature, and (3) low-temperature eclogites are more likely to contain free rutile. Only about a third of the ∼250 samples considered here would have whole-rock TiO2 contents (reconstructed with calculated rutile modes) required for rutile saturation during subduction and partial melting. If there is a role for subducting oceanic crust now sampled as mantle eclogite, the characteristic Ti–Nb–Ta depletion in continental crust may require fluid-dominated processes, where these elements are not efficiently mobilised. In garnet, Ti uptake on the octahedral site is accommodated primarily by coupled substitution with Na and subordinately with a divalent metal cation, and there is no evidence for substitution on the tetrahedral site. For samples equilibrated to the conductive geotherm, Ti in addition to Na enrichment may be indicative of equilibration in the diamond stability field. The jadeite component in clinopyroxene as a function of temperature is a good indicator of the geotherm to which the various samples equilibrated, and can be used to reveal samples within each suite that have been affected by isobaric heating. The distribution of V in eclogitic garnet, clinopyroxene and rutile is affected by bulk composition, temperature and oxygen fugacity. In carefully vetted, low-temperature samples with TiO2 contents &amp;gt;0·8 wt%, V-based oxybarometry may monitor redox conditions prevailing during metamorphism of oceanic crust or, at lower TiO2, during (secular) cooling-related exsolution of rutile from garnet or clinopyroxene, whereas in higher-temperature ilmenite-bearing samples metasomatic conditions may be recorded.

A Devonian arc–back-arc basin system in the southern Chinese Altai: Constraints from geochemical and Sr-Nd-Pb isotopic data for meta-basaltic rocks

The early to middle Paleozoic accretionary history of the Chinese Altai orogen remains controversial, especially along its southern margin facing the Paleo-Asian Ocean. This study conducts a systematic investigation on the Devonian meta-volcanic sequences from the Altay Formation in the southern Chinese Altai to constrain their petrogenesis and tectonic implications. LA-ICP-MS zircon UPb dating for rocks from the bottom and top layers of the studied volcanic sequences indicates their eruption age at ca. 380 Ma. The basaltic rocks are low-K tholeiitic with high TiO2 (average = 1.98 wt%), and have rare-earth element (REE) patterns similar to the mid-ocean ridge basalt (MORB), in spite of a large variation in light REE (LREE). They also show enrichment in large ion lithophile elements (LILE; e.g. Ba, U, and Th) and depletion in high field strength elements (HFSE; e.g. Nb and Ta), which are hallmarks of supra-subduction zone basaltic magma. Thus, the elemental data indicate that these rocks are compositionally intermediate between MORB and island-arc tholeiite (IAT).The basaltic samples have positive εNd(t) values (+3.1 to +10.3) and elevated initial 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotopic ratios. Whole-rock elemental and isotopic data suggest that the rocks were likely derived from a dominant N-MORB type depleted asthenosphere mantle, which was variably metasomatized by or interacted with the relatively enriched sub-arc mantle. In combination with the field relationship and regional geology, our study suggests that the meta-basaltic rocks from the middle Altay Formation formed in a mature back-arc spreading environment. We propose an evolving tectonic scenario from a magmatic arc to arc rifting, and finally back-arc opening in the southern Chinese Altai, which corresponds to an extensional regime and resultant asthenosphere upwelling event in this region during Devonian time, probably in response to a trench retreat.

Initiation of modern-style subduction in the Neoarchean: From plume to subduction with frequent slab break-off

Abstract Fundamental geodynamic changes from vertical tectonics to lateral subduction occurred during the Neoarchean, yet detailed processes related to this transition and initiation of modern-style subduction remain enigmatic. Successive Neoarchean magmatic rocks including both plume-derived komatiites and subduction-related supracrustal and intrusive rocks appeared and preserved key information on the late Archean geodynamic changes in the Western Shandong Province granite-greenstone belt (WSP), North China Craton. In this study, whole-rock geochemical and Sm-Nd isotopic data and zircon U-Pb and Lu-Hf isotopes are reported for early Neoarchean supracrustal and intrusive rocks for the WSP. Temporally, the early Neoarchean magmatic movements in the WSP can be subdivided into two stages, including the early stage (2.77–2.69 Ga) and the late stage (2.69–2.60 Ga). Spatially, from southwest to northeast, intrusive rocks with similar ages define three belts (A, B, and C). Early stage tholeiitic and enriched meta-basalts were plume-related, representing oceanic crust opening from a pre-early Neoarchean continent. Slab subduction at least initiated at ca. 2.74 Ga and generated various Neoarchean tonalite-trondhjemite-granodiorites, quartz diorites, and arc-related volcanic rocks and mafic intrusions. Episodic emergence of meta-basaltic rocks and/or mafic intrusions with depleted εHf(t) values and low (La/Yb)N ratios indicates frequent slab break-offs during ca. 2.70–2.68 Ga, 2.66–2.64 Ga, and 2.62–2.60 Ga due to a relatively hotter mantle and regional heating by mantle plume. Secular geochemical changes of mafic and felsic rocks in this study outline roles of slab subduction in contributions of cooling the mantle, secular mantle refertilization, and crustal growth.

Petrological Characteristic and Whole Rock Geochemistry of Metamorphic Rocks in Melangè Complex of Ciletuh Area, West Java, Indonesia

Melangí¨ Complex of Ciletuh Area, West Java, Indonesia is located at 106 ° 23 '38.9 "- 106 ° 25' 27.6" E and 7 ° 14 '27.6 "- 7 ° 12' 46.2" S. This area has unique geological features where the Early Cenozoic uplifted subduction rocks such as ophiolites and metamorphic rocks are exposed. This study aimed to determine the condition of the stratigraphic position and geological structure of metamorphic rock unit in the field, to find out the metamorphic facies and their evolution, to know the parental rocks (protoliths) of metamorphic rocks, and to interpret tectonic environments for the formation of metamorphic rocks based on stratigraphy, geological structure, petrography, and geochemistry. Study of literatures, geological mapping, petrology and geochemical analyses were used as methods of this research. The entire analysis is combined as a guidance in interpretation of petro-tectonic environment. Distribution of metamorphic rock outcrops found in several places, such as, Citisuk River, Cikopo River and Cikarikil River. Based on petrography analysis, metamorphic rocks types consist of schistose amphibolite, various type of greenschist, phyllite and quartzite. The protolith of metamorphic rocks in Ciletuh is quite diverse, namely metasediments such as pelitic, psammitic, calc-silicate sediment, and meta-igneous such as gabbro and basalt. The presence of epidote, chlorite, and calcite in schistose amphibolite show retrograde metamorphism process in lower temperature-pressures condition. The occurance of quartz and calcite vein in several samples was shown as an indication of hydrothermal alteration. Based on geochemical characteristics, the result showed that sedimentary source of metasedimentary rocks were derived from the volcanic arc environment. While, metabasalt rocks were originated from the Island Arc tectonic environment. According to the association, these metamorphic rocks were formed by regional metamorphism, as the result of subduction process and orogenic event. Thus, retrograde metamorphism indicates the lifting or accretion process that caused by subsecuent tectonic activity.

Ages and geochemistry of Early Jurassic granitoids in the Lesser Xing’an–Zhangguangcai Ranges, NE China: Petrogenesis and tectonic implications

Abstract Early Jurassic granitoids are widespread in the Lesser Xing’an–Zhangguangcai Ranges, providing excellent targets to understand the late Paleozoic to early Mesozoic tectonic framework and evolution of Northeast China, especially the Jiamusi block and its related structural belts. In this paper, we present new geochronological, geochemical, and isotopic data from the granitoids in the Lesser Xing’an–Zhangguangcai Ranges to constrain the early Mesozoic tectonic evolution of the Mudanjiang Ocean between the Jiamusi and Songnen blocks. Our results show that the granitic intrusions in the Lesser Xing’an–Zhangguangcai Ranges are mainly composed of syenogranite, monzogranite, granodiorite, and tonalite, which have crystallization ages from 196 to 181 Ma. Their geochemical features indicate that these Jurassic intrusions are all high-K calc-alkaline I-type granites with metaluminous to weakly peraluminous compositions. These granitoids are characterized by enrichments in large ion lithophile elements (e.g., Ba, Th, U) and light rare earth elements and depletions in high field strength elements (e.g., Nb and Ta) and heavy rare earth elements, which are typical for continental arc–type granites. The sources of these granitoids were likely derived from juvenile Mesoproterozoic to Neoproterozoic crustal materials (e.g., metabasaltic rocks). Integrated with data from regional coeval magmatism, metamorphism, metallogeny, and structure, our new data suggest that the granitoids in the Lesser Xing’an–Zhangguangcai Ranges were probably formed in an active continental margin setting, which fits well in our previous model of Early Jurassic westward subduction of the Mudanjiang Ocean between the Jiamusi and Songnen blocks.

Behavior of calcium isotopes during continental subduction recorded in meta-basaltic rocks

In order to investigate the behavior of Ca isotopes during subduction, we report high precision Ca isotopic data of a suit of well-characterized meta-basaltic rocks from the Dabie-Sulu orogen, including six greenschists, six amphibolites and seven eclogites. Except two samples that may have been affected by carbonation alteration, greenschists, amphibolites and eclogites yield comparable δ44/42Ca values ranging from 0.36 to 0.41‰, 0.32 to 0.39‰ and 0.32 to 0.42‰, respectively. Overall, Dabie-Sulu meta-basaltic rocks have an average δ44/42Ca of 0.37 ± 0.06‰ (2SD, N = 17/19), comparable to terrestrial basalts previously reported (0.36 ± 0.05‰, 2SD; N = 34). Combined with no correlation of δ44/42Ca with H2O and Rb/TiO2, this study suggests that prograde metamorphism dehydration cannot significantly fractionate Ca isotopic compositions of meta-basaltic rocks. Given the comparable CaO contents in meta-basaltic rocks with increasing metamorphism grade, isotope fractionation may be limited by insignificant Ca loss during dehydration. Two heavy greenschists with δ44/42Ca of 0.46‰ and 0.49‰ also have high CO2 concentrations, indicating carbonation alteration that is evidenced by 0.04M HCl leaching experiments. The limited δ44/42Ca variation observed in the other metamorphic samples is most likely inherited from their igneous protoliths. Considering that terrestrial basalts have δ44/42Ca systematically lower than the upper mantle, subduction of mafic lithologies (e.g., oceanic slabs) thus could reflux light Ca isotopes back into the mantle and may create mantle heterogeneity.

Age and genesis of the Neoarchean Algoma-type banded iron formations from the Dengfeng greenstone belt, southern North China Craton: Geochronological, geochemical and Sm–Nd isotopic constraints

Banded iron formations (BIFs) are important for understanding the composition and redox condition of paleo-oceans, as well as the evolution of the Precambrian crust. The Neoarchean Dengfeng greenstone belt (ca. 2.54–2.50 Ga) in the southern North China Craton (NCC) consists of metamorphosed igneous and sedimentary assemblages including some BIFs. Here, we present mineralogical, geochronological, geochemical and Sm–Nd isotopic data of the Dengfeng BIFs for the first time to constrain their age, origins and source characteristics, and then discuss their paleo-environmental implications. The BIFs are mainly associated with metabasaltic rocks and quartz-rich schists, and can be classified as Algoma-type BIFs. Zircon U-Pb dating of quartz-rich schists interbedded with the BIFs, coupled with ages of wall-rock metabasites, demonstrate that the Dengfeng BIFs formed at ~2.53–2.51 Ga. Most BIF samples have high contents of SiO2 and Fe2O3T, and extremely low contents of Al2O3, TiO2, and high-field strength elements (e.g., Zr, Hf, Th, U), indicating that they are detrital-free deposits. In the Post-Archean Australia Shale normalized multi-element diagram, the detritus-free BIF samples are characterized by depleted light rare earth elements, positive La and Y anomalies, and suprachondritic Y/Ho ratios, resembling geochemical features of modern seawater. The absence of negative Ce anomaly indicates an anoxic condition. Positive Eu anomalies and relatively high concentrations of transition metal elements suggest that high-temperature hydrothermal fluids contributed to the source of BIFs. Isotopically, all BIF samples have positive εNd(t) values (+0.12 to + 4.53), consistent with the range of those (+0.10 to + 4.31) of ca. 2.66–2.50 Ga Dengfeng metabasites and TTG gneisses, suggesting input of possible juvenile crustal fluxes. The elemental and isotopic characteristics of the Dengfeng BIFs collectively suggest that they precipitated from anoxic seawater with various input of hydrothermal fluids and oceanic crust-derived fluxes.

Element and Sr–O isotope redistribution across a plate boundary-scale crustal serpentinite mélange shear zone, and implications for the slab-mantle interface

Fluid flow along crustal plate boundary-scale serpentinite shear zones may provide insights into element transfer at one of Earth's most important but least accessible lithological boundaries: the slab-mantle wedge plate interface. For ∼55 km in the Southern Alps of New Zealand, a 10 to 100s m wide block-in-matrix serpentinite mélange shear zone – the Livingstone Fault – separates the harzburgitic base of the Dun Mountain Ophiolite Belt from arc-derived metasedimentary and minor metabasaltic rocks. Transects across the shear zone reveal systematic chemical and isotopic changes over several hundred meters, from harzburgite (<1 wt% H2O, 87Sr/86Sr = 0.7031–0.7045, δ18O=∼+5.3‰) to massive and then foliated mélange serpentinite (up to 13.7 wt% H2O, 87Sr/86Sr = 0.7084, δ18O=+7.8‰). These changes require hydration of peridotite by fluids derived from, or that interacted with, the metasediment-dominated wallrock (87Sr/86Sr = 0.7073–0.7093, δ18O≥+9.6‰). Tremolite-veined serpentinite and metasomatized greyschist blocks (87Sr/86Sr = 0.7063–0.7071; δ18O=+8.7) within the serpentinite mélange also require ultramafic- and metasediment–derived element mixing. Halogens (Br and I) were enriched in serpentinized ultramafic rocks. By analogy with the crustal Livingstone Fault, the slab-mantle plate interface may have a strongly anisotropic basal serpentinite mélange shear zone that grades into a thicker zone of massive and variably serpentinized peridotite with a progressively decreasing “slab” element and isotope signature. Permeability in subduction-related shear zones is predicted to be mainly parallel to the slab but channelized fluid mobility across the Livingstone Fault was aided by transient fracturing. Oxidation of harzburgite during serpentinization appears to have been accompanied by mobilization of Si, Ca, Mg, Sr, Br, I, Ga, Mo, Ta, W, V and Y. As many of these elements occur in the sources of arc magmas, serpentinization may play an important role in mobilizing them. The Livingstone Fault lithologies and their element and isotope transfer records means this shear zone may represent a field-accessible analogue for the shallow slab-mantle plate interface.

Petrogenesis of Early Silurian granitoids in the Longshoushan area and their implications for the extensional environment of the North Qilian Orogenic Belt, China

Analyses of the zircon U—Pb age, mineralogical chemistry and whole-rock geochemistry of the Jiling pluton along the southern margin of the Alxa Block were conducted. The results of these analyses will provide crucial information for investigations of Paleozoic crust-mantle interactions in the North Qilian Orogenic Belt (NQOB) and explorations of the transition age from a compressional to extensional environment during the collision between the Alxa Block and the Qilian-Qaidam Block. The zircon dating results reveal that the Jiling pluton is composed of Early Silurian granitoids (435–442 Ma) with contemporaneous mafic microgranular enclaves (MMEs). Most granitoids contain amphibole and titanite and have A/CNK values <1, illustrating that they belong to amphibolite-rich calc-alkaline granitoids (ACGs) and K-rich and K-feldspar porphyritic calc-alkaline granitoids (KCGs). Both the granitoid and MME samples display variable initial Sr and Nd isotopic compositions. We suggest that the Jiling pluton was formed by crust-mantle-derived magma mixing. The high HFSE content of the MMEs indicates that the mantle-derived magmas were derived from melting of the mantle wedge metasomatized by adakitic magma, and the crustal-derived magmas were derived from melting of Paleoproterozoic meta-basaltic rocks in the Longshoushan Group. The discovery of the Jiling granitoids indicates that the NQOB was dominated by a compressional environment during Middle Ordovician-Early Silurian in response to the collision between the Qilian-Qaidam Block and the Alxa Block, and the initial extensional environment began before 442 Ma.

Late Triassic Biotite Monzogranite from the Western Litang Area, Yidun Terrane, SW China: Petrogenesis and Tectonic Implications

The Late Triassic igneous rocks in the Yidun terrane can provide vital insights into the evolution of Plaeo‐Tethys in western China. We present new zircon U–Pb, whole‐rock geochemistry, and Sr–Nd–Pb–Hf isotopic data for the Litang biotite monzogranites, Yidun terrane. The biotite monzogranites have a zircon U‐Pb age of 206.1±1.0 Ma (MSWD=1.9, n=30), which indicates Late Triassic magmatism. The biotite monzogranites display I‐type affinity, high Na2O (3.38–3.60wt%) contents, medium SiO2(67.12–69.13wt%), and low P2O5 contents (0.10–0.12wt%). They are enriched in Rb, Th, and Ba and depleted in Nb and Ta, with negative Eu anomalies (Eu/Eu∗=0.74–0.81). They have evolved Sr‐Nd‐Pb‐Hf isotopic composition, i.e., (87Sr/86Sr)i=0.714225 to 0.714763, negative εNd(t) values of –2.0 to –2.6 with two‐stage Nd model ages ranging from 1.01 to 1.05 Ga, negative εHf(t) values of –3.4 to –4.1 with two‐stage Hf model ages of 1.85 to 1.88 Ga, suggesting a matured crustal sources. Their low Al2O3/TiO2 ratios and medium Cao/Na2O ratios, medium Mg# and SiO2 contents, low [molar Al2O3/(MgO+FeOT)] values, and high [molar Cao/(MgO+FeOT)] values indicate that the Litang biotite monzogranite was formed by partial melting of metabasaltic rocks. Based on the previous studies, we propose that the Litang biotite monzogranite derived from the westward subduction and closure of the Ganzi–Litang ocean during the Late Triassic. The mantle wedge‐derived mafic melts provided sufficient heat for partial melting of ancient metabasalt protolith within the middle‐lower crust.

Paleozoic tectonothermal event in Mt. Dongbatu, Dunhuang terrane, southernmost Central Asian Orogenic Belt (CAOB): Implications for petrogenesis and geological evolution

In this contribution, systematic petrographical, whole-rock geochemical, geochronological and zircon Lu-Hf isotopic investigations were conducted on Paleozoic (ca. 410–400 Ma) metamorphic-magmatic rocks from Mt. Dongbatu in the central Dunhuang terrane. The metamorphic rocks in the Shangkouzi area are of meta-calcsilicate rocks, and granitoid rocks occurring within these rocks are geochemically of trondhjemite. Whereas, metamorphic rocks in the Gangou area are metabasaltic rocks. LA-ICP MS zircon U-Pb dating results reveal that these metamorphic-magmatic rocks recorded ca. 1.6 Ga magmatic event and metamorphic events of ca. 410–400 Ma and ca. 370 Ma. Field occurrence, whole-rock geochemistry, mineral composition and zircon Hf isotopes suggest that the 1.6 Ga mantle-derived basaltic magma, together with iron-rich and Ca-rich components and mica-quartz schist of Paleoproterozoic supracrustal rocks probably contributed to protolith of meta-calcsilicate rocks in the Shangkouzi area, and granitoid rocks within meta-calcsilicate rocks were possibly generated by partial melting of calcsilicate rocks.Combined with previously published data, it is suggested that the protoliths of Paleozoic high-grade metamafic rocks in the Dunhuang terrane are predominantly composed at least of basaltic rocks produced by magmatic events of 2.5–1.8 Ga, ca. 1.8 Ga and ca. 1.6 Ga. The Paleozoic granitoid rocks in the Dunhuang terrane might have been derived from different magma sources as partial melting of subducted plate, or thickened lower continental crust, or pre-existing basaltic rocks produced by 1.6 Ga and 1.8 Ga magmatic activities, and calcsilicate rocks genetically associated basaltic magmatism at ca. 1.6 Ga. Considering regional tectonic evolution, it can be concluded that the Dunhuang terrane was probably in a post-collisional extension setting at ca. 1.6 Ga, producing certain amount of basaltic and calcsilicate rocks. From the beginning of Silurian (ca. 440 Ma), the Dunhuang terrane was involved in the orogenic processes related to the closure of the southern Paleo-Asian Ocean, becoming the active continental margin of the southernmost CAOB.

Rheology and stress in subduction zones around the aseismic/seismic transition

Subduction channels are commonly occupied by deformed and metamorphosed basaltic rocks, together with clastic and pelagic sediments, which form a zone up to several kilometers thick to depths of at least 40 km. At temperatures above ~ 350 °C (corresponding to depths of > 25–35 km), the subduction zone undergoes a transition to aseismic behavior, and much of the relative motion is accommodated by ductile deformation in the subduction channel. Microstructures in metagreywacke suggest deformation occurs mainly by solution-redeposition creep in quartz. Interlayered metachert shows evidence for dislocation creep at relatively low stresses (8–13 MPa shear stress). Metachert is likely to be somewhat stronger than metagreywacke, so this value may be an upper limit for the shear stress in the channel as a whole. Metabasaltic rocks deform mainly by transformation-assisted diffusional creep during low-temperature metamorphism and, when dry, are somewhat stronger than metachert. Quartz flow laws for dislocation and solution-redeposition creep suggest strain rates of ~ 10−12 s−1 at 500 °C and 10 MPa shear stress: this is sufficient to accommodate a 100 mm/yr. convergence rate within a 1 km wide ductile shear zone.The up-dip transition into the seismic zone occurs through a region where deformation is still distributed over a thickness of several kilometers, but occurs by a combination of microfolding, dilational microcracking, and solution-redeposition creep. This process requires a high fluid flux, released by dehydration reactions down-dip, and produces a highly differentiated deformational fabric with alternating millimeter-scale quartz and phyllosilicate-rich bands, and very abundant quartz veins. Bursts of dilational microcracking in zones 100–200 m thick may cause cyclic fluctuations in fluid pressure and may be associated with episodic tremor and slow slip events. Shear stress estimates from dislocation creep microstructures in dynamically recrystallized metachert are ~ 10 MPa.

Permo-Triassic evolution of the southern margin of the Central Asian Orogenic Belt revisited: Insights from Late Permian igneous suite in the Daheishan Horst, NE China

The Daheishan Horst in Jilin Province, NE China, is the key geological unit that links the Solonker-Xar Moron-Changchun and Changchun-Yanji sutures, which are generally interpreted to mark the zone of closure of the Paleo-Asian Ocean along the southeastern margin of the Central Asian Orogenic Belt (CAOB). Here we investigate a suite of volcanic rocks from Daheishan and the Doushantouzi syenogranite intrusion to gain insights into the Permian–Triassic tectonic evolution of the eastern segment of the Paleo-Asian Ocean. Zircon U-Pb data and zircon rare earth element (REE) patterns indicate that the igneous suite formed during the Late Permian (ca. 253–256Ma) and underwent late-stage alteration during Early Triassic tectono-thermal events. The presence of older magmatic zircon grains in this igneous suite suggest multiple pulses of magmatism during the Permian in this region. The Daheishan volcanic rocks predominantly consist of intermediate-felsic rocks, including andesite, rhyodacite and rhyolite with minor basaltic lava. Geochemically, the basaltic rocks belong to calc-alkaline series, whereas the intermediate-felsic rocks classify as calc-alkaline to high-K calc-alkaline series. The Doushantouzi syenogranites belong to peraluminous I-type granites, with the A/CNK ratio between 1.06 and 1.15. All these rocks have an arc-like affinity with enriched light rare earth elements (LREE) and large ion lithophile elements (LILE; e.g. Rb, Ba and U) and depleted high field strength elements (HFSE; e.g. Nb, Ta and Ti). The Daheishan intermediate-felsic rocks and Doushantouzi syenogranites have higher SiO2 contents but lower MgO contents and Mg# values and plot within the field of experimentally derived partial melts from metabasaltic rocks in MgO vs. SiO2 diagram. These geochemical features, together with the positive εNd(t) (+1.6 to +4.6) and εHf(t) (+1.53 to +7.41) values of zircon grains, indicate that the primary magma of these intermediate-felsic rocks likely originated from the partial melting of a juvenile metabasaltic lower crust. In contrast, the basaltic lavas were probably derived from the partial melting of a depleted mantle wedge that was metasomatized by fluids from a subducted slab, as suggested by their low initial 87Sr/86Sr ratios and depleted εNd(t) (+3.6 to +4.4) values. These data, in conjunction with regional geological investigations, suggest that formation of this Late Permian igneous suite was related to the northward subduction of the Paleo-Asian oceanic plate beneath the Songliao-Xilinhot block and that the eastern segment of the Paleo-Asian Ocean did not close before the Late Permian.

Lead isotope evolution across the Neoproterozoic boundary between craton and juvenile crust, Bayuda Desert, Sudan

Metaigneous mafic and ultramafic rocks from the juvenile Neoproterozoic Arabian Nubian Shield (ANS) and the Proterozoic, reworked Saharan Metacraton (SMC) have been analysed for major- and trace elements and Sr, Nd, and Pb isotopes. Most of the rocks are amphibolites metamorphosed at amphibolite facies conditions, some with relicts of a granulite facies stage. The other rocks are metapyroxenites, metagabbros, and some ultramafic rocks. Trace element compositions of the metabasaltic (dominantly tholeiitic) rocks resemble the patterns of island arcs and primitive lavas from continental arcs. Variable Sr and Nd isotope ratios indicate depleted mantle dominance for most of the samples. 207Pb/204Pb signatures distinguish between the influence of high 207Pb/204Pb old SMC crust and depleted mantle signatures of the juvenile ANS crust. The Pb isotope signatures for most metabasaltic rocks, metapyroxenites and metagabbros from SMC indicate an autochthonous formation. The interpretation of the new data together with published evidence from mafic xenoliths on SMC and ophiolite from ANS allows an extrapolation of mantle evolution in time. There are two lines of evolution in the regional mantle, one, which incorporates potential upper crust material during Neoproterozoic, and a second one with a depleted mantle signature since pre-Neoproterozoic that still is present in the Red Sea and Gulf of Aden spreading centres.

MINERALOGICAL, PETROLOGICAL AND GEOCHEMICAL FEATURES OF THE UNIQUE LAPIS LACEDAEMONIUS (KROKEATIS LITHOS) FROM LACONIA, GREECE: APPROACH ON PETROGENETIC PROCESSES WITHIN THE TRIASSIC VOLCANIC CONTEXT

The Lapis Lacedaemonius (krokeatis lithos) is a well-known meta-volcanic rock of great historical importance. Petrographic observations, mineral chemistry data, as well as geochemical analysis of selected samples, reveal that these rocks are porphyritic metabasaltic rocks which have been significantly affected by saussuritization and also by restricted silicification processes. They represent subduction related calc-alkaline volcanic rocks which also appear in the adjacent Hellenic Triassic volcanic outcrops, and appear to be associated with the rift/drift phase within the Pindos oceanic realm. The unique features of the Lapis Lacedaemonius, when compared to geochemically similar volcanic rock outcrops, are mainly attributed to their distinct porphyritic textures, predominantly with microlithically textured groundmass along with the coarse grained plagioclase, and to saussuritization processes. The Lapis Lacedaemonius seems to have been formed in a sub-volcanic system closely associated with epidosites, suggesting that metasomatism occurred within hydrothermal upflow zones.

Different stages of chemical alteration on metabasaltic rocks in the subduction channel: Evidence from the Western Tianshan metamorphic belt, NW China

To understand the geochemistry of subduction zone metamorphism, especially the large-scale mass transfer at forearc to subarc depths, we carried out a detailed study of a ∼1.5m size metabasaltic block with well-preserved pillow structures from the Chinese Western Tianshan high- to ultrahigh-pressure metamorphic belt. This metabasaltic block is characterized by omphacite-rich interiors gradually surrounded by abundant channelized (veins) glaucophane-rich patches toward the rims. The glaucophane-rich rims share the same peak metamorphic conditions with omphacite-rich interiors, but have experienced stronger blueschist-facies overprinting during exhumation. Representative samples from the glaucophane-rich rims and omphacite-rich interiors yield a well-defined Rb-Sr isochron age of 307±23Ma, likely representing this overprinting event. Both glaucophane-rich rims and omphacite-rich interiors show elevated K-Rb-Cs-Ba-Pb-Sr contents relative to their protolith, reflecting a large-scale enrichment of these elements and formation of abundant phengite during subduction. Compared with the omphacite-rich interiors, the glaucophane-rich rims have gained rare earth elements (REEs, >25%), U-Th (∼75%), Pb-Sr (>100%) and some transition metals like Co and Ni (25–50%), but lost P (∼75%), Na (>25%), Li and Be (∼50%); K-Rb-Cs-Ba show only 10% loss. These chemical changes would be caused by serpentinite-derived fluids during the exhumation in the subduction channel. Therefore, there are two stages of fluid action in the subduction channel. As the formation of phengite stabilizes K-Rb-Cs-Ba at the first stage, the residual fluids released from the phengite-rich metabasaltic rocks would be depleted in these elements, which are unlikely to contribute to elevated contents of these elements in arc magmas if phengite remains stable at subarc depths. In addition, the decrease of U/Pb ratios as the preferred enrichment of Pb over U in the eclogitic rocks during the first stage chemical alteration may further lead to the lower radiogenic Pb isotope component of the deeply subducted ocean crust with time, which is inconsistent with the high radiogenic Pb isotope component of high µ (=238U/204Pb) basalts.

Corrigendum: Earth's first stable continents did not form by subduction.

This corrects the article DOI: 10.1038/nature21383.

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

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