Subduction Of Oceanic Crust Research Articles

Classifying east coasts of Hormozgan province using Shepard method and satellite imagery

Coastal Makran in the southern part of Iran is one of the most attractive coastal areas in the world which was under geological and geomorphological attention from long ago. This area has been studied extensively from various points of view. Coastal classification however, was never attributed to the area so far. Categorizing the east coast of Hormozgan province therefore, located at the south-eastern coastline of Iran (entrance of Coastal Makran), using satellite imageries of ASTER and ETM+ sensors in the context of Shepard classification is considered as the subject of this investigation. The two satellite imageries derived from the two sensors were compared with each other using Arc GIS environment. Some statistical deviations were found when comparing the results derived from the two sensors. According to the both imageries however, the terrestrial factors (primary agents) were dominant in the area (over 90%), followed by the maritime factors (secondary agents) (about 5%). The coverage of the man-made coasts was only 1% of the total area. This conclusion was predictable; while, according to the literature the Makran zone was formed by the subduction of oceanic crust of Arabian Plate under the Eurasian Plate implying the area has a terrestrial background.

Geochemistry, petrology and geodynamic setting of the Urumieh plutonic complex, Sanandaj–Sirjan zone, NW Iran: New implication for Arabian and Central Iranian plate collision

The Urumieh plutonic complex, in the northernmost part of Sanandaj-Sirjan zone (SSZ) of Iran, consists of ten basic-acidic units which formed in response to subduction and continental collision of the SSZ with the Arabian plate to the south during Mid-Late Cretaceous times. Geochemically, the plutonic unit is divided into three distinct groups: I-type, S-type and A-type that mainly belong to calc-alkalic series. The I-type intrusions, especially mafic members, are enriched in LREE and LILE and possibly formed from metasomatized mantle wedge during the subduction of the Neo-Tethys oceanic crust beneath the SSZ. The felsic I-type rocks are depleted in Ba, Sr, Nb, Ta, Ti and Eu, but enriched in Rb, Th, K, Ce, U and La. These data suggest that they formed in deep crustal levels via partial melting of crustal sources by injection of hot mantle magmas. The S-type rocks are characterized by low Na2O (<3.02 wt%), high LILE, relatively high values of molar Al2O3/(MgO+FeO) and K2O/Na2O ratios combined with low CaO/(MgO+FeO*) ratios. These features show that the S-type granites originated from partial melting of a metapelitic to metagreywacke source. The A-type alkali feldspar granites formed through the slab break off after the continental collision in northwestern Iran by decompression melting of crustal protolith. The author's new model implies that collision between Arabian margin and north SSZ initiated in the Late Cretaceous and completed until Late Paleocene. In contrast, in the southeast, subduction was active during this period of time, but collision presumably occurred during the Middle to Late Miocene.

Recycling of palaeo-Pacific subducted oceanic crust related to a Fe–Cu–Au mineralization in the Xu-Huai region of North Anhui-Jiangsu, East China: Geochronological and geochemical constraints

ABSTRACTIn this study, Early Cretaceous skarn deposits and genesis of their host diorite/monzodiorite porphyry in the Xuzhou-Huaibei (Xu-Huai) region, northern Anhui-Jiangsu have been discussed by detailed geochemical work. In-situ zircon U–Pb dating of the diorites related to Fe–Cu–Au deposits shows that they were formed between 131.4 ± 1.5 Ma and 130.8 ± 1.8 Ma. Geochemical data indicate a depletion of high field strength elements (HFSE) in the diorite porphyry with similarity to that of arc-related igneous rocks. The diorite porphyry was probably derived from typical arc magmas related to continental margin subduction characterized by light rare earth elements (LREEs) enrichment and HFSE depletion. REEs compositions of apatite in the diorite porphyry indicate that the dioritic magma was produced from the metasomatized subcontinental mantle by slab-derived fluids. The magma was proven to be a high oxygen fugacity; thus, it was particularly conducive to the precipitation of Fe, Cu, Au and other ore-forming elements. The δ34S values of pyrite and chalcopyrite of Fe–Cu–Au ores range from −0.2‰ to 2.8‰, indicating that the sulphur in the ore was probably derived from deep-seated magmas. Integrated with geochronological and geochemical analyses, we suggest that the Early Cretaceous igneous suites associated with Fe–Cu–Au deposits in the Xu-Huai region are related to recycling subduction of Pacific oceanic crust.

Permian arc evolution associated with Panthalassa subduction along the eastern margin of the South China block, based on sandstone provenance and U–Pb detrital zircon ages of the Kurosegawa belt, Southwest Japan

We have studied the petrography, geochemistry, and detrital zircon U–Pb ages of sandstones from shallow-marine forearc sediments, accretionary complexes (ACs), and metamorphosed accretionary complexes (Meta-ACs) within the Kurosegawa belt of Southwest Japan. Those rocks formed in a forearc region of a Permian island arc associated with subduction of the Panthalassa oceanic crust along the eastern margin of the South China block (Yangtze block). The provenance of the shallow-marine sediments was dominated by basaltic to andesitic volcanic rocks and minor granitic rocks during the late Middle to Late Permian. The ACs were derived from felsic to andesitic volcanic rocks during the Late Permian. The provenance of Meta-ACs was dominated by andesitic volcanic rocks in the Middle Permian. The provenance, source rock compositions, and zircon age distribution for the forearc sediments, ACs and Meta-ACs have allowed us to reconstruct the geological history of the Permian arc system of the Kurosegawa belt. During the Middle Permian, the ACs were accreted along the eastern margin of the South China block. The Middle Permian arc was an immature oceanic island arc consisting of andesitic volcanic rocks. During the Late Permian, the ACs formed in a mature arc, producing voluminous felsic to andesitic volcanic rocks. A forearc basin developed during the late Middle to Late Permian. Subsequently, the Middle Permian ACs and part of the Late Permian AC underwent low-grade metamorphism in the Late to Early Jurassic, presenting the Meta-ACs.

Geology and Lead–Sulphur Isotope Compositions of the Tongyu Volcanic‐Hosted Massive Sulphide Copper Deposit in the Western Part of the North Qinling Orogen, China

AbstractThe Tongyu copper deposit, located in the western part of the North Qinling Orogen, China, is one of several volcanic‐hosted massive sulphide (VHMS) deposits with industrial value and is also a typical example of mineralization related to the subduction and metallogenesis during the Caledonian orogeny. We conducted systematic lead–sulphur isotope geochemical analyses of the Tongyu deposit to understand the possible ore‐forming material sources and tectonic settings. Twenty‐six sulphide samples yielded clustered δ34SCDT values of 1.13‰–3.36‰, average 2.22‰, and show a tower‐type distribution, implying that the sulphur of the Tongyu copper deposit mainly originated from a mantle source. The Pb isotope compositions of sulphides (206Pb/204Pb = 17.59225–18.56354, average 18.32020; 207Pb/204Pb = 15.51770–15.69381, average 15.66217; 208Pb/204Pb = 37.99969–39.06953, average 38.52722) are close to the values of the volcanic host rocks (206Pb/204Pb = 18.10678–18.26293, average 18.21158; 207Pb/204Pb = 15.63196–15.68188, average 15.65345; 208Pb/204Pb = 38.43676–38.56360, average 38.49171), thus consistent with the Pb in ores and volcanic host rocks having been derived from a common source that was island‐arc Pb related to oceanic crust subduction. The northward subduction of the Palaeo–Qinling oceanic crust triggered dehydration of the slab, which generated a large amount of high‐oxygen‐fugacity aqueous hydrothermal fluid. The fluid rose into the mantle wedge, activated and extracted metallogenic material and promoted partial melting of the mantle wedge. The magma and ore‐forming fluid welled up and precipitated, finally forming the Tongyu VHMS copper deposit.

Geochronology, geochemistry, Sr–Nd–Hf isotopic compositions, and petrogenetic and tectonic implications of Early Cretaceous intrusions associated with the Duolong porphyry–epithermal Cu–Au deposit, central Tibet

ABSTRACTThis article reports new zircon laser ablation-multicollector-inductively coupled plasma-mass spectrometry U–Pb and Hf isotope, whole-rock major and trace element, and Sr–Nd isotope data for mineralized and barren intrusions associated with the Duolong porphyry–epithermal copper–(gold) deposit (DPCD, a mining camp containing several individual deposits) in the western Qiangtang Terrane (QT), central Tibet. These data are used to further our understanding of the geological evolution of this region. The mineralized and barren DPCD intrusions are typical I-type granitoids that were synchronously emplaced at ca. 112.6–125.9 Ma. These igneous rocks show arc affinities that are characterized by enrichments in the light rare earth elements (LaN/YbN = 4.08–15.23) and the light ion lithophile elements (Rb, Th, U, K, and Pb), and depletions in the high field strength elements (Nb, Ta, and Ti). They have 87Sr/86Sr(i) values of 0.7046–0.7079, Nd(t) values of –6.0 to +1.1, and two-stage Nd model ages of ca. 823–1410 Ma. Zircons from these intrusive rocks have variable but generally positive εHf(t) values (–2.7 to +13.7) and relatively young zircon Hf crustal model ages of 335–1351 Ma. Combining these data with geochemical data reported in recent studies, we infer that the mineralized and barren DPCD intrusions formed in a continental marginal arc setting and likely originated from a common parental magma that was result of magma mixing of juvenile crust-derived basaltic melts and old lower crust-derived melts. The formation of the DPCD intrusions indicates that the Bangongco–Nujiang oceanic lithosphere was still undergoing northward subduction beneath the western QT at ca. 112.6–125.9 Ma, suggesting in turn that the oceanic basin have not closed completely during the Early Cretaceous. These new data also indicate that the processes that occur during the subduction of oceanic crust in continental marginal arc settings produce and preserve juvenile crustal material, leading to net continental crust vertical growth and thickening.

Zircon LA-ICP-MS Dating and Geochemical Characteristics of I-type Granitoids from the Yanhu Area, West Segment of the Bangongco-Nujiang Suture (Western Tibet): Petrogenesis and Implications for the Southward Subduction of the Tethyan Ocean

ABSTRACT The Yanhu granitoids are located in the west segment of the Bangongco-Nujiang suture in the western Tibetan Plateau. The main rock types of the granitoids are diorite porphyry, quartz diorite, granodiorite, granite and granite porphyry. Here, their zircon LA-ICP-MS U-Pb ages and petrogeochemical data are reported. Three groups of magmatic events can be distinguished from the Yanhu area: group 1 includes samples AK01 and ZK01 of diorite porphyry, and sample D3658 of quartz diorite that yield mean zircon U-Pb ages of 121.0 ± 2.7 Ma, 116.6 ± 2.0 Ma and 116.0 ± 3.9 Ma, respectively; group 2 includes sample D0050 of diorite porphyry, samples D1393 and D3660 of granodiorite and sample D3065 of granite porphyry that yield mean zircon U-Pb ages of 104.9 ± 2.0 Ma, 105.4 ± 3.8 Ma, 104.2 ± 1.9 Ma and 104.2 ± 1.9 Ma, respectively; group 3 includes sample D3093 of granite that yields mean zircon U-Pb ages of 93.6 ± 1.5 Ma. The zircon LA-ICP-MS U-Pb ages suggest that the Yanhu granitoids were emplaced at 121.0 – 93.6 Ma, representing Cretaceous magmatism in the west segment of the Bangongco-Nujiang suture. The granitoids are composed of SiO2 (56.57 to 76.98 wt.%), Al2O3 (12.20 to 17.90 wt.%), Na2O (3.61 to 4.98 wt.%), K2O (2.06 to 4.71 wt.%) and CaO (0.27 to 5.74 wt.%). The Yanhu granitoids exhibit enrichment in LREE (light REE) and LILE (large ion lithophile elements) such as Rb, Th, U, Pb and K and depletion of HREE (heavy REE), P, Ti, Nb, Ta and Zr. Their A/CNK ratios of 0.85-1.06 are &amp;lt;1.1, implying that they are high-K, metaluminous-weakly peraluminous I-type granites. The Yanhu granitoids were generated mainly by partial melts of the meta-igneous lower crust and some arc-related materials. The Yanhu granitoids probably formed in VAG and syn-COLG tectonic settings related to the southward subduction of the Tethyan Ocean. Diorite porphyry and quartz diorite magmatism from 121.0 Ma to 116.0 Ma may be associated with the southward Bangongco–Nujiang Tethys oceanic crust subduction. Diorite porphyry, granodiorite, and granite porphyry magmatism from 105.4 Ma to 104.2 Ma may be associated with the rising asthenosphere induced by the slab breakoff. Granite magmatism from 93.6 Ma may be related to the crustal thickening induced by the final amalgamation of the Lhasa Terrane and the Qiangtang Terrane.

Early Cretaceous adakitic rocks in the Anqing region, southeastern China: constraints on petrogenesis and metallogenic significance

ABSTRACTThe Anqing region in Lower Yangtze River metallogenic belt is one of the important Cu polymetal producers in China. The origin of Cu polymetallic deposits in the region is closely related to Early Cretaceous adakitic intrusions. To constrain the petrogenetic and metallogenic significance of the adakitic rocks, a detailed geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic study was performed. The Anqing adakitic rocks (SiO2 = 57.4–64.2 wt.%) consist mainly of quartz monzodiorite, formed at 138.2 ± 1.7 Ma (Mean Standard Weighted Deviation (MSWD) = 0.61). They have high MgO, Al2O3, Sr, and low Rb, Y, Yb contents, together with high Sr/Y (50.5–222) and La/Yb (31.9–46.9) ratios. They also show negative whole-rock εNd(t) (−9.8 to −8.5) and zircon εHf(t) (−10.0 to −5.4), and high oxygen fugacity (mainly −17.0 to −8.01) values and radiogenic Pb isotopic compositions with (206Pb/204Pb)i = 17.692–17.884, (207Pb/204Pb)i = 15.413–15.511, and (208Pb/204Pb)i = 37.611–37.943. Coupled with negative Nb–Ta anomalies, low K2O/Na2O ratios (0.39–0.62), and high Mg# values (0.44–0.71), these data suggest the adakitic rocks and associated large-scale Cu–Au mineralization of the Anqing region resulted from partial melting of the high oxidized subducted oceanic crust. Addition of mantle-derived magmas and assimilation of crustal materials during emplacement are also possible.

Pulses of Late Mesozoic magmatism: Zircon ages and Hf-O isotopic composition of the Qingyang-Jiuhuashan granitic complex, southern Anhui province, eastern China

Late Mesozoic magmatic rocks are widely distributed in southern Anhui province, and are represented by the Qingyang-Jiuhuashan granitic complex, which consists dominantly of the Qingyang pluton of granodiorite and monzogranite, and the Jiuhuashan pluton of K-feldspar granite. These two plutons have zircon U-Pb ages of 138Ma–128Ma. Combining the new data with previously reported age data, the magmatism in southern Anhui province can be subdivided into three stages: early stage of ∼152–136Ma, middle stage of ∼136–130Ma, and latest stage of ∼130–120Ma. Granodiorite (early stage) and monzogranite (middle stage) of the Qingyang pluton have SiO2 contents of 66–68wt%, total alkali contents of 7.5–8.6wt%, K2O/Na2O ratios of 1.1–1.3, A/CNK ratios of 0.9–1.1, and low FeOT/(FeOT+MgO) values of 0.71–0.73. These rocks show depletion in Nb, Ta, and Zr and slightly negative Eu anomalies. K-feldspar granite (late stage) of the Jiuhuashan pluton is characterized by high SiO2 contents (>75wt%), K2O/Na2O ratios of 1.1–1.3, and A/CNK ratios of 0.9–1.2, and high FeOT/(FeOT+MgO) values of 0.88–0.93. They have high concentrations of Nb, Ta, and Zr, and strongly negative Eu anomalies. Initial εHf values of zircon grains from the granodiorite and monzogranite range from −8.01 to −6.71 and −11.40 to −9.43, with δ18O values varying from 7.16‰ to 8.04‰ and 7.26‰ to 8.06‰, respectively. Zircon grains from K-feldspar granites have relatively high initial εHf values of −7.62 to −4.78, but similar δ18O values of 7.03–8.84‰. Geochemical features of granodiorite and monzogranite point to an I-type signature, whereas those of K-feldspar granite show an A-type affinity. We propose that the granodiorite and monzogranite originated from mixing melts produced by partial melting of lithospheric mantle metasomatized by fluids and crustal rocks induced by underplating basaltic magmas, related to subduction of the Paleo-Pacific oceanic crust, whereas the K-feldspar granite formed from crustal anatexis in an extension setting caused by rollback of the subduction.

Neogene palaeogeography and basin evolution of the Western Carpathians, Northern Pannonian domain and adjoining areas

The data on the Neogene geodynamics, palaeogeography, and basin evolution of the Western Carpathians, Northern Pannonian domain and adjoining areas (ALCAPA Mega-unit) are summarized, re-evaluated, supplemented, and newly interpreted. The proposed concept is illustrated by a series of palinspastic and palaeotopographic maps. The Miocene development of the Outer Carpathians reflects the vanishing subduction of the residual oceanic and/or thinned continental crust. A compression perpendicular to the front of the orogenic system led to the closing of residual flysch troughs and to accretionary wedge growth, as well as to the development of a foredeep on the margin of the European Platform. Docking of the Outer Western Carpathians accretionary wedge, together with the Central Western Carpathians and Northern Pannonian domain, was accompanied by stretching of the overriding microplate. An orogen parallel and perpendicular extension was associated with the opening and subsidence of the Early and Middle Miocene hinterland (back-arc) basin system that compensated counter-clockwise rotations of the individual crustal fragments of ALCAPA. The Late Miocene development relates to the opening of the Pannonian Basin System. This process was coupled with common stretching of both ALCAPA and Tisza-Dacia Mega-units due to the pull exerted by subduction rollback in front of the Eastern Carpathians. The filling up of the hinterland basin system was associated with thermal subsidence and was followed by the Pliocene tectonic inversion and consequent erosion of the basin system margins, as well as part of the interior.

The behavior of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from Western Alpine ophiolites

AbstractArc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidized or oxidizing components from the subducting slab to the mantle wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chloride, and carbonate ligands, to remark on the chemistry and oxidation state of fluids released during prograde metamorphism of subducted oceanic crust. We present data for metagabbros and metabasalts from the Chenaillet massif, Queyras complex, and the Zermatt‐Saas ophiolite (Western European Alps), which have been metamorphosed at typical subduction zone P‐T conditions and preserve their prograde metamorphic history. There is no systematic, detectable fractionation of either Fe or Zn isotopes across metamorphic facies, rather the isotope composition of the eclogites overlaps with published data for MORB. The lack of resolvable Fe isotope fractionation with increasing prograde metamorphism likely reflects the mass balance of the system, and in this scenario Fe mobility is not traceable with Fe isotopes. Given that Zn isotopes are fractionated by S‐bearing and C‐bearing fluids, this suggests that relatively small amounts of Zn are mobilized from the mafic lithologies in within these types of dehydration fluids. Conversely, metagabbros from the Queyras that are in proximity to metasediments display a significant Fe isotope fractionation. The covariation of δ56Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction.

کانه زایی مس- نقره سولفید توده ای آتشفشان زاد نوع بشی در توالی آتشفشانی- رسوبی کرتاسه پسین: مثال موردی کانسار گرماب پایین، جنوب شرق شاهرود

کانسار مس- نقره گرماب پایین در توالی آتشفشانی- رسوبی کرتاسه پسین زیر پهنه سبزوار واقع ‌شده است. کانه زایی به شکل هندسی چینه‌سان و چینه‌کران در افق چینه‌ای خاص به همراه نهشته‌های برون‌دمی رخ‌داده است. سنگ میزبان کانه زایی را سنگ‌های آتشفشانی آندزیتی- داسیتی و آذرآواری‌های وابسته تشکیل می‌دهند. کانه زایی از کمرپایین به سمت کمربالا در چهار رخساره رگه- رگه‌چه ای، توده‌ای- نیمه توده‌ای، لایه‌ای و رسوبات برون‌دمی رخ‌داده است. کانی‌شناسی ماده معدنی شامل کانی‌های اولیه نظیر پیریت، مگنتیت و کالکوپیریت و کانی‌های ثانویه نظیر مس طبیعی، مالاکیت، کوپریت، کولیت و اکسیدهای آهن- منگنز است. دگرسانی سنگ دیواره، به‌طور عمده از نوع کلریتی و به مقدار کمتر شامل سیلیسی، آرژیلیتی و زئولیتی است. حداکثر عیار طلا و نقره در کانسار به ترتیب 1 و 19 گرم در تن است. به نظر می‌رسد که بروز فعالیت آتشفشانی زیردریایی در یک حوضه پشت‌کمانی در کرتاسه پسین، به تشکیل این کانسار سولفید توده‌ای نوع بشی منجر شده است.

Petrochemistry of Khunrang intrusive complex, southeast of Kerman, Iran: Implications for magmatic evolution of Sanandaj-Sirjan zone in the Mesozoic time

A noticeable characteristic of the Sanandaj-Sirjan zone (SSZ) of Iran is the presence of extensive mafic to felsic intrusive igneous rocks in the host metamorphic rocks of Palaeozoic and Mesozoic eras. A similar composition is evident in batholithic size Khunrang Instrusive Complex (KIC) of southern SSZ. The rocks that make the KIC complex are mostly leucocratic microdiorite, quartz diorite, tonalite, granodiorite, granite and subordinate mesocratic to melanocratic pyroxene hornblende-gabbro and microgabbro. Field evidence and geochemical data suggest that the felsic rocks are not the products of fractional crystallization from a mafic phase. Using various discrimination and normalized multi-element diagrams suggest that mafic rocks, of tholeiitic to calc-alkaline affinities, were formed in an island arc or continental arc setting, from a metasomatized lithospheric mantle, above the stability field of garnet in a subduction zone environment. The felsic rocks, calc-alkaline and metaluminous in nature, have I-type granite characteristics. Their relative enrichment in large ion lithophile elements (LILEs) such as Ba, Cs and K and depletion in high field strength elements (HFSEs) such as Nb, Ta and Ti, is a signature of their development in an arc related environment in an active continental margin, similar to KIC mafic rocks. Geochemical characteristics suggest that the KIC felsic rocks were formed by partial melting of metabasic rocks of lower crust in response to underplating of mantle-derived basaltic magmas in an active continental margin as a result of Neo-Tethys oceanic crust subduction beneath the Central Iranian microcontinent in Mesozoic time.

Geological and geochemical constraints on the Cheshmeh-Frezi volcanogenic stratiform manganese deposit, southwest Sabzevar basin, Iran

The Cheshmeh-Frezi Mn deposit belongs to the southwest Sabzevar basin to the north of the Central Iranian microcontinent. This basin, which hosts abundant mineral deposits including Mn exhalative and Besshi-type Cu-Zn volcanogenic massive sulfide deposits, followed an evolution closely related to the subduction of the Neo-Tethys oceanic crust beneath the Central Iranian microcontinent. Two major sedimentary sequences are recorded within this basin: (I) the Lower Late Cretaceous volcano-sedimentary sequence (LLCVSS) and (II) the Upper Late Cretaceous sedimentary dominated sequence (ULCSS). The Cheshmeh-Frezi Mn deposit is hosted within red tuff with interbeds of green tuffaceous sandstone of the LLCVSS. Mineralization occurs as stratiform blanket-like and tabular orebodies. Psilomelane, pyrolusite and braunite are the main minerals of the ore, which display a variety of textures. Such as layered, laminated, disseminated, massive, replacement or open space fillings. The footwall and hanging-wall volcanic rocks are predominantly andesite and trachyandesite rocks. Footwall and hangingwall volcanic rocks at Cheshmeh-Frezi are enriched in light rare earth elements (LREEs) compared to chondrite, have steep REE patterns, and generally show Ta and Nb depletions relative to chondrite which are characteristic of back-arc environments. The significant geochemical characteristics of ore such as high Mn content (12.41–33.14 wt%; average 19.41 wt%), low concentration of Fe (0.64–2.27 wt%; average 1.63 wt%), high Ba (49.7–9901ppm, average 2728.67ppm), LREE>HREE, and negative Ce and Eu anomalies reveal a primary distal hydrothermal-exhalative source for mineralization. Cheshmeh-Frezi deposit, in comparison with different types of volcanogenic manganese deposits shows broad similarities with the Cuban-type Mn deposits such as tectonic, host and associated rock types, geometry, textures, structures, mineralogy and lithogeochemistry.

The roles of pyroxenite and peridotite in the mantle sources of oceanic basalts

Subduction of oceanic crust generates chemical and lithological heterogeneities in the mantle. An outstanding question is the extent to which these heterogeneities contribute to subsequent magmas generated by mantle melting, but the answer differs depending on the geochemical behaviour of the elements under investigation: analyses of incompatible elements (those that preferentially concentrate into silicate melts) suggest that recycled oceanic crust is an important contributor, whereas analyses of compatible elements (those that concentrate in crystalline residues) generally suggest it is not. Recently, however, the concentrations of Mn and Ni—two elements of varying compatibility—in early-crystallizing olivines, have been used to infer that erupted magmas are mixtures of partial melts of olivine-rich mantle rocks (that is, peridotite) and of metasomatic pyroxene-rich mantle rocks (that is, pyroxenite) formed by interaction between partial melts of recycled oceanic crust and peridotite. Here, we test whether melting of peridotite alone can explain the observed trend in olivine compositions by combining new experimental data on the partitioning of Mn between olivine and silicate melt under conditions relevant to basalt petrogenesis with earlier results on Ni partitioning. We show that the observed olivine compositions are consistent with melts of fertile peridotite at various pressures—importantly, melts from metasomatic pyroxenites are not required. Thus, although recycled materials may well be present in the mantle source regions of some basalts, the Mn and Ni data can be explained without such a contribution. Furthermore, the success of modelling the Mn–Ni contents of olivine phenocrysts as low-pressure crystallization products of partial melts of peridotite over a range of pressures implies a simple new approach for constraining depths of mantle melting.

Seismic evidence for Earth's crusty deep mantle

Seismic tomography resolves anomalies interpreted as oceanic lithosphere subducted deep into Earth's lower mantle. However, the fate of the compositionally distinct oceanic crust that is part of the lithosphere is poorly constrained but provides important constraints on mixing processes and the recycling process in the deep Earth. We present high-resolution seismic array analyses of anomalous P-waves sampling the deep mantle, and deterministically locate heterogeneities in the lowermost 300 km of the mantle. Spectral analysis indicates that the dominant scale length of the heterogeneity is 4 to 7 km. The heterogeneity distribution varies laterally and radially and heterogeneities are more abundant near the margins of the lowermost mantle Large Low Velocity Provinces (LLVPs), consistent with mantle convection simulations that show elevated accumulations of deeply advected crustal material near the boundaries of thermo-chemical piles. The size and distribution of the observed heterogeneities is consistent with that expected for subducted oceanic crust. These results thus suggest the deep mantle contains an imprint of continued subduction of oceanic crust, stirred by mantle convection and modulated by long lasting thermo-chemical structures. The preferred location of the heterogeneity in the lowermost mantle is consistent with a thermo-chemical origin of the LLVPs. Our observations relate to the mixing behaviour of small length-scale heterogeneity in the deep Earth and indicate that compositional heterogeneities from the subduction process can survive for extended times in the lowermost mantle.

Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U–Pb and Lu–Hf isotopes

Constraints on the composition of the depleted mantle Sm–Nd and Lu–Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since >4Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle.Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.

What goes down comes back up.

Geochemistry Earth's oceans are thought to have come from a slow outgassing of water from the mantle during planet formation. However, we also know that volatiles can be recycled during subduction of ocean crust. Kendrick et al. turned to a suite of geochemical data to look at the impact of recycling on volatile outgassing, which turns out to dominate the volatile budget. Their results point to a heavily processed mantle in which volatiles are introduced by subduction and released as crust dehydrates. This discovery implies an early Earth in which surface water was recycled into the mantle and outgassed back onto the surface, without the need to invoke deep mantle reservoirs. Nat. Geosci. 10.1038/ngeo2902 (2017).

Petrogenesis and tectonic implications of the charnockites in the Yishui Terrane, North China Craton

A combined study of zircon U-Pb ages, Lu-Hf isotopes and whole-rock geochemistry was carried out for representative charnockites from the Yishui terrane in the Western Shandong Province, NCC. The results demonstrate that the charnockites in the Yishui terrane are of igneous origin rather than derivation from remelting or metamorphism of supracrustal rocks. Zircon U-Pb dating for the charnockites in the Yishui terrane indicates protolith emplacement around 2.52–2.56Ga and synchronous metamorphism around 2.50–2.53Ga. Zircon Lu-Hf isotopes analyses show that the charnockites have positive εHf(t) values with an average +4.03, and depleted mantle two-stage zircon Hf model ages (TDM2) mainly cluster around 2.7–3.0Ga, indicative of a source of juvenile crust formed around 2.7–3.0Ga. Furthermore, the charnockites with <70% wt% SiO2 tend to be magnesian calcic to calc-alkalic and metaluminous, only a small population of the charnockites with >70wt% SiO2 are ferroan alkali-calcic and weakly peraluminous, which are consistent with geochemical features of Cordilleran-type granitoids, suggesting a deeply eroded magmatic arc derived from remelting of juvenile crust mainly formed around 2.7–3.0Ga. Taken the comprehensives of available whole-rock geochemical and Zircon Lu-Hf isotopic data of the charnockites, synchronous emplacement and metamorphism, high-temperature setting of charnockites, granulite-facies metamorphism of anticlockwise P-T path in the Yishui terrane as well as the whole NCC, we conclude that the combined geodynamics of mantle plume and oceanic crust subduction might contribute to the formation of the charnockites in the Yishui terrane as well as similar late Archean charnockites in other areas of the NCC, further implying extensive occurrences of modern-style plate tectonic accretions in late Archean in the NCC.

Paleo‐Tethyan Oceanic Crust Subduction in the Eastern Section of the East Kunlun Orogenic Belt: Geochronology and Petrogenesis of the Qushi'ang Granodiorite

AbstractThe Qushi'ang granodiorite (QSG) is located at the central east of the ophiolitic melange belt in the East Kunlun Orogenic Belt (EKOB) in the northern margin of the Qinghai‐Tibetan Plateau. LA‐MC‐ICP‐MS zircon U–Pb dating suggests that the granodiorite and mafic microgranular enclaves (MMEs) crystallized 246.61±0.62 and 245.45±0.9 Ma ago, respectively. Granodiorite, porphyritic diorite, and MMEs are metaluminous and medium‐K calk ‐ alkaline series, with island‐arc magma features, such as LILE enrichment and HFSE depletion. The porphyritic diorite has high Cr (13.50 ppm to 59.01 ppm), Ni (228.53 ppm to 261.29 ppm), and Mg# (46–54). Granodiorite and porphyritic diorite have similar mineral compositions and evolved major and trace elements contents, particularly Cr and Ni, both of which are significantly higher than that in granites of the same period. The crystallization age of MMEs is close to that of granodiorite, and their major and trace elements contents are in‐between porphyritic diorite and granodiorite. The results suggest that the original mafic magma, which was the product of mantle melting by subduction process, intruded into the lower crust (Kuhai Rock Group), resulting in the formation of granodiorite. Countinous intrusion of mafic magma into the unconsolidated granodiorite formed MMEs and porphyritic diorite. The granodiorite reformed by late‐stage strike‐slip faulting tectonic event indicates that the strike‐slip fault of Middle Kunlun and the collision of the Bayanhar block with East Kunlun were later than 246 Ma. Therefore, the formation of the QSG not only indicates the critical period of evolution of East Kunlun but also represents the tectonic transition from oceanic crust subduction to slab breaking.