Early Paleozoic Oceanic Crust Research Articles

Munabulake ophiolitic mélange: Implication for the evolution history of the north branch of the Proto-Tethys ocean

One of the ophiolites that record the Proto-Tethys Ocean’s episodic closure is the Munabulake ophiolitic mélange, which is situated in the middle of the Kunlun–Qaidam and Altun–Qilian blocks. Detailed field mapping revealed that the Munabulake ophiolitic mélange comprises local (ultramafic rocks, basalts, andesites, gabbros, diorites, and plagiogranites) and exotic (marble, gneiss, schist, and amphibolite) blocks, many of which are in the schist matrix (Qimantage Group). Based on geochronological, geochemical, and petrological observations, the mafic rocks in the Munabulake ophiolitic mélange can be categorized into three categories: 498-Ma OIB-like gabbros, 468-Ma Hawaiian alkaline basalt-like dolerite and pillow basaltic slices, and 428 Ma massive SSZ-like ultramafic rocks. The 501–452 Ma I-type granites exhibit arc affinities due to the oceanic crust subduction. These findings, along with spatial relationships, suggest that the Early Paleozoic ophiolite complex, island arc rocks, and accretionary complex generated as an intra-oceanic arc system as a result of obduction of the south Altun Ocean’s onto the Central Altun block within a north-directed subduction event. A dextral strike–slip was evident throughout the Early Paleozoic oceanic crust subduction based on the whole set of planar and linear structural data, and the subduction polarity was likely to the north. According to the ophiolitic mélange’s youngest rocks and the existence of 413 Ma granite dykes that are widely exposed in the Munabulake ophiolitic mélange, the Munabulake ophiolitic mélange was most likely emplaced during the Middle Silurian. This Munabulake ophiolitic mélange is similar in age and petrochemical characteristics to the other ophiolites in the South Altun subduction–collision belt, indicating that the Manabulak ophiolite mélange is a westward extension of the Apa–Mangya subduction–collision belt, which formed during the northward subduction of the South Altun Ocean slab during the Early Paleozoic. Thus, the final closing time of the South Altun Ocean is between 413 and 428 Ma.

The Early Silurian Gabbro in the Eastern Kunlun Orogenic Belt, Northeast Tibet: Constraints on the Proto-Tethyan Ocean Closure

The early Paleozoic is a crucial period in the formation and evolution of the Eastern Kunlun Orogenic Belt (EKOB), and is of great significance for understanding the evolutionary history of the Proto-Tethyan Ocean. This paper presents new petrography, geochemistry, zircon U–Pb dating, and Lu–Hf isotopic research on the Yuejingshan gabbro from the eastern segment of the EKOB. Zircon U–Pb data suggests that the gabbro formed in the Early Silurian (435 ± 2 Ma). All samples have relatively low TiO2 contents (0.45–2.97%), widely varying MgO (6.58–8.41%) and Mg# (58–65) contents, and are rich in large ion lithophile elements (LILE such as Rb, Ba, Th, and U) and light rare earth elements (LREE). This indicates that it has a similar geochemical composition to island arc basalt. The major element features indicate that the formation of this gabbro underwent fractional crystallization of clinopyroxene, olivine, and plagioclase. The depletion of high field strength elements (HFSE, such as Nb, Ta, and Ti), and a slightly positive Hf isotope (with εHf(t) ranging from 1.13 to 2.45) may be related to the partial melting of spinel-bearing peridotite, led by slab fluid metasomatism. The gabbro likely represents magmatic records of the latest period of the early Paleozoic oceanic crust subduction in the Eastern Kunlun. Therefore, the final closure of the Proto-Tethyan Ocean and the beginning of collisional orogeny occurred before the Early Silurian.

Petrology and Geochemistry of the lawsonite (pseudomorph)-bearing eclogite in Yuka terrane, North Qaidam UHPM belt: An eclogite facies metamorphosed oceanic slice

Although lawsonite eclogite is predicted to be common at depth of ~45–300km in subduction zones, it is rarely exposed at Earth's surface. In this study, we identified a lawsonite (pseudomorph)-bearing eclogite in Yuka terrane, North Qaidam UHPM belt. It is mainly composed of garnet, amphibole, clinozoisite, quartz and rutile with minor omphacite relics and talc. Petrographic investigation identified abundant rectangular- or rhombic- shaped mineral aggregates composed of polycrystalline clinozoisite or polyphase clinozoisite+paragonite±phengite±quartz and clinozoisite+amphibole±quartz±Na-rich plagioclase occurring in garnet porphyroblasts. The peculiar shape, mineral assemblages and reconstructed compositions of these aggregates show that they are pseudomorphs after lawsonite. Phase equilibrium modeling predicts that the lawsonite (pseudomorph)-bearing eclogite experienced a clockwise P–T evolution with a peak stage at 570°C, 27kbar, near the quartz–coesite transition boundary. Geochemical data show that the lawsonite (pseudomorph)-bearing eclogite has a MORB-like affinity with flat Chondrite–normalized rare earth element (REE) distribution patterns. Zircons from this sample have similar δ18O values (5.38±0.10‰) with the normal mantle (5.3±0.3‰), indicating that the protolith of the lawsonite (pseudomorph)-bearing eclogite is derived directly from mantle rock without continental crust contamination. The MORB-like character and zircon O isotopic composition of the lawsonite (pseudomorph)-bearing eclogite indicate that its protolith is likely to be an oceanic crustal slice. SIMS and LA–ICP–MS zircon U–Pb dating yielded a 436–438Ma eclogite-facies metamorphic age for the lawsonite (pseudomorph)-bearing eclogite, similar to the continental subduction age of the North Qaidam UHPM belt, but did not obtain its protolith age. These data indicate that the protolith of the lawsonite (pseudomorph)-bearing eclogite may be either a piece of Proterozoic ocean that was emplaced in the active continental margin of the Qaidam block during the amalgamation of the Rodinia supercontinent, or the early Paleozoic oceanic crust in the transition zone between the Qilian Ocean and the Qaidam block.

The Permian Dongfanghong island-arc gabbro of the Wandashan Orogen, NE China: Implications for Paleo-Pacific subduction

The Dongfanghong hornblende gabbro is located in the western part of the Wandashan Orogen and to the east of the Jiamusi Block in NE China. It was emplaced into Early Paleozoic oceanic crust (i.e. Dongfanghong ophiolite) at ~275Ma and both later collided with the eastern margin of the Jiamusi Block. The Dongfanghong gabbro is sub-alkaline with high Na2O contents and is characterized by enrichment in light rare earth elements (LREE), large ion lithosphile elements (LILE), Sr, Eu, and Ba, and depletion in high field strength elements (HFSE). The enriched isotopic signatures (87Sr/86Sri=~0.7065, εNd(t)=~−0.5, 208Pb/204Pbi=~38.05, 207Pb/204Pbi=~15.56, 206Pb/204Pbi=~18.20 and zircon εHf(t)=~+5.8) indicate an enriched mantle (EM2) source, with some addition of continental material. It has arc geochemical affinities similar to Permian arc igneous rocks in the eastern margin of the Jiamusi Block, the Yakuno Ophiolite in SW Japan, arc rocks along the western margin of the North America Craton, and also the Gympie Group in eastern Australia. All these features, together with information from tectonic discrimination diagrams, suggest that the Dongfanghong gabbro formed in an immature island arc. The spatial configuration of ~290Ma immature continental arc rocks in the eastern part of the Jiamusi Block and the ~275Ma immature island arc Dongfanghong gabbro in the Wandashan Orogen to the east is best explained by eastward arc retreat and slab roll-back of the Paleo-Pacific Plate. This model is also supported by the Carboniferous–Permian stratigraphic transition in the Jiamusi Block from oceanic carbonate rocks to coal-bearing terrestrial clastic rocks and andesites. We thus suggest that both Paleo-Pacific subduction and roll-back occurred in the Early Permian along the eastern margin of Asia.

The multi-stage tectonic evolution of the Xitieshan terrane, North Qaidam orogen, western China: From Grenville-age orogeny to early-Paleozoic ultrahigh-pressure metamorphism

The geodynamic evolution of the early Paleozoic ultrahigh-pressure metamorphic belt in North Qaidam, western China, is controversial due to ambiguous interpretations concerning the nature and ages of the eclogitic protoliths. Within this framework, we present new LA-ICP-MS U–Pb zircon ages from eclogites and their country rock gneisses from the Xitieshan terrane, located in the central part of the North Qaidam UHP metamorphic belt. Xitieshan terrane contains clearly different protolith characteristics of eclogites and as such provides a natural laboratory to investigate the geodynamic evolution of the North Qaidam UHP metamorphic terrane. LA-ICP-MS U–Pb zircon dating of three phengite-bearing eclogites and two country rock gneiss samples from the Xitieshan terrane yielded 424–427Ma and 917–920Ma ages, respectively. The age of 424–427Ma from eclogite probably reflects continental lithosphere subduction post-dating oceanic lithosphere subduction at ~440–460Ma. The 0.91–0.92Ga metamorphic ages from gneiss and associated metamorphic mineral assemblages are interpreted as evidence for the occurrence of a Grenville-age orogeny in the North Qaidam UHPM belt. Using internal microstructure, geochemistry and U–Pb ages of zircon in this study, combined with the petrological and geochemical investigations on the eclogites of previous literature’s data, three types of eclogitic protoliths are identified in the Xitieshan terrane i.e. 1) Subducted early Paleozoic oceanic crust (440–460Ma), 2) Neoproterozoic oceanic crust material emplaced onto micro-continental fragments ahead of the main, early Paleozoic, collision event (440–420Ma) and 3) Neoproterozoic mafic dikes intruded in continental fragments (rifted away from the former supercontinent Rodinia). These results demonstrate that the basement rocks of the North Qaidam terrane formed part of the former supercontinent Rodinia, attached to the Yangtze Craton and/or the Qinling microcontinent, and recorded a complex tectono-metamorphic evolution that involved Neoproterozoic and Early Paleozoic orogenies.

Petrogenesis and implications for tectonic setting of Cambrian suprasubduction-zone ophiolitic rocks in the central Beishan orogenic collage, Northwest China

The Hongliuhe ophiolite is a fragment of the early Paleozoic oceanic crust situated in a suture zone central to the Beishan orogenic collage, which is a subset to the Central Asian Orogenic Belt in Northwest China. It has lithologic, geochemical and chronological similarities to the Yueyashan–Xichangjing ophiolite 400km east, along strike, allowing for correlative interpretations of marginal seafloor spreading processes. The Hongliuhe ophiolite is explored in this study with complete petrologic, geochemical and structural descriptions. The ultramafic body comprises cumulate-layered lherzolite and gabbro plutonic lithologies, tectonized with syn-magmatic extension. A U–Pb age of 520.3±5.8Ma was obtained by SHRIMP analysis of zircons from a pegmatitic gabbro. The ultramafic rocks are variably depleted and some show refertilization of LILE’s consistent with slab dehydration in a supra-subduction mantle wedge. Mineral chemistry shows high Cr# (36–82) populations of spinel, and wide-ranging lower Mg# (8–50), consistent with known forearc peridotite chemistry partially re-equilibrated by tectonism or through staged magmatism. Olivine and pyroxene chemistries are consistent with this interpretation. A thin volcanic cover is interlayered with red cherts, and has a bimodal set of geochemical signatures, both island-arc tholeiite and calc-alkaline. They bear distinct Nb–Ta anomalies consistent with slab-derived fluid initiated melting in a supra-subduction environment. Coarse volcaniclastic turbidites infilled paleo-topography along the ophiolite, and they are interbedded with quartzofeldspathic sandstones, indicating an initial proximity to an arc. The sandstones contain detrital spinel with analyses showing relation to both the ophiolite and possibly a secondary alkalic volcanic source. The supracrustal sequence continues with a thick, contiguous covering of margin-related rocks, including interlayered limestone, pyroclastic and siliciclastic rocks, and basalts. As a constraint on emplacement we provide an U–Pb Zircon SHRIMP date (413.6±3.5Ma) from an undeformed granitoid body intruded within the overlying stratigraphy. The entire analysis paints a picture of subduction rollback-related extension affecting the margin of an emerging or mature arc just prior to a known magmatic cycle that ended with a collision in the Devonian at the latest. This suture is one of many in the Beishan, and our interpretation provides evidence in a long running debate supporting accretionary orogenesis in an island-arc archipelago as the main system for growing sialic crust in the Paleozoic central Asia.

Blueschist metamorphism and its tectonic implication of Late Paleozoic–Early Mesozoic metabasites in the mélange zones, central Inner Mongolia, China

Blueschists in central Inner Mongolia are distributed as layers and blocks in mélanges including the southern zone in Ondor Sum area and the northern zone in Manghete and Naomuhunni areas. They have been attributed to the subduction of Early Paleozoic oceanic crust. Blueschists from Ondor Sum and Naomuhunni are characterized by occurrence of sodic amphibole coexisting with epidote, albite, chlorite, calcic amphibole (in Ondor Sum) and muscovite (in Naomuhunni). Blueschists in Manghete contain porphyroblastic albite with inclusions of garnet and epidote in a matrix dominated by calcic–sodic amphibole, epidote, chlorite, albite and muscovite. Phase equilibria modeling for three blueschist samples using pseudosection suggest that the AlM2 contents in sodic amphibole can be used as a good barometer in the limited assemblage involving sodic amphibole+actinolite+epidote+chlorite+albite+quartz under pressures <4–6kbar, while this barometer is largely influenced by temperature and bulk Fe2O3 contents in the actinolite-absent assemblage sodic amphibole+epidote+chlorite+albite+quartz of higher pressure and the AlM2 contents are not pressure-controlled in the albite-absent assemblage sodic amphibole+epidote+chlorite+quartz under pressures>7–10 kbar. In the sodic amphibole-bearing assemblages, the NaM4 contents in sodic amphibole mainly decrease as temperature rises, being a potential thermometry. The calculated pseudosections constrain the P–T conditions of blueschists to be 3.2–4.2kbar/355–415°C in Ondor Sum, 8.2–9.0kbar/455°C–495°C in Manghete and 6.6–8.1kbar/420–470°C in Naomuhunni. These P–T estimates indicate a rather high geothermal gradient of 18–25°C/km for the blueschist metamorphism, being of intermediate P/T facies series. Available zircon U–Pb age data suggests that the protoliths of blueschists were formed later than Late Paleozoic–Early Mesozoic and metamorphosed soon afterwards. An alternative interpretation for the tectonic implication of blueschists in central Inner Mongolia is that they may be a new type attributed to closure of limited ocean basins and do not represent a tectonic regime occurred in conventional subduction setting.

Underplating generated A- and I-type granitoids of the East Junggar from the lower and the upper oceanic crust with mixing of mafic magma: Insights from integrated zircon U–Pb ages, petrography, geochemistry and Nd–Sr–Hf isotopes

Whole rock major and trace element, Nd–Sr and zircon Hf isotopic compositions and secondary-ion mass spectrometry zircon U–Pb ages of eleven granitoid intrusions and dioritic rocks from the East Junggar (NW China) were analyzed in this study. The East Junggar granitoids were emplaced during terminal Early to Late Carboniferous (325–301Ma) following volcanic eruption of the Batamayi Formation. Zircons from the East Junggar granitoids yielded 210 concordant 206Pb/238U ages which are all younger than 334Ma and exhibit εHf(t) values distinctly higher than Devonian arc volcanic-rocks. Seismic P-wave velocities of deep crust of the East Junggar proper resemble those of oceanic crust (OC). These characteristics suggest absence of volcanic rock and volcano-sedimentary rock of Devonian and Early Carboniferous from the source region. The East Junggar granitoids show εNd(t) and initial 87Sr/86Sr values substantially overlapping those of the Armantai ophiolite in the area. The Early Paleozoic OC with seamount-like composition as the Zhaheba–Armantai ophiolites remained in the lower crust and formed main source rock of the East Junggar granitoids. Based on petrography and geochemistry, the East Junggar granitoids are classified into peralkaline A-type in the northern subarea, I-type (I1 and I2 subgroups) mainly in the north and A-type in the south of the southern subarea. The perthitic or argillated core and oligoclasic rim with an argillated boundary of feldspar phenocrysts and inclusion of perthites or its overgrowth by matrix plagioclase, in the monzogranites (northern subarea), suggest mixing of peralkaline granitic magma with mafic magma. In the north of the southern subarea, the presence of magmatic microdioritic enclaves (MMEs) in the I1 subgroup granitoids, transfer of plagioclase phenocrysts and hornblendes between host granodiorite and the MME across the boundary and a prominent resorption surface in the plagioclase phenocrysts indicate mixing of crustal magma (I2 subgroup granitoids) with mafic magma. Magma mixing shifted (87Sr/86Sr)i of the I1 subgroup granitoids towards the mantle array. Two generations of hornblende with zonal distribution and similar mineral and geochemical compositions of quartz monzodiorite and hosted MME with unfractionated rare earth elements (REE) suggest extended magma mixing with onset probably at or near source region. These observations imply concurrency of mantle input and the crustal melting and, hence, a causal relationship between underplating/intraplating and the lower OC/upper OC melting. The I-type granitoids experienced plagioclase and hornblende fractionations, whereas fractionated phases of the two groups of A-type granites were alkali feldspar and albite–oligoclase with significant involvement of F−-rich fluid. Granodioritic parent magmas of the I2 subgroup granitoids stemmed from the hydrous upper OC. Parent magmas of the two A-type groups possess syenogranitic or quartz syenitic compositions. The peralkaline A-type granites stemmed from the lower OC, whereas the A-type granites from dehydrated upper OC left behind after extensive partial melting and extraction of I-type granitoids. Based on comparison in the ternary system Mg2SiO4–CaAl2SiO6–SiO2, most of the Batamayi volcanic rocks with affinity to ocean-island basalts were derived from asthenospheric upwelling. The gabbro–dioritic rocks with higher light to heavy REE ratios stemmed from metasomatized lithospheric mantle. Both of the above mafic rocks contain subducted slab component.Loci of ponding of mafic magma and crustal melting migrated upward and southeastward from the lower OC in the northern subarea (yielding the peralkaline A-type granites) to the upper OC in the southern subarea. The latter evolved from hydrous (yielding the I-type granitoids in the north of the subarea) to dehydrated (yielding the A-type granites in the south of the subarea). Sequence of these events resulted in increasingly-later formation of the East Junggar granitoids southeastwards with an age span of ~20Ma. Minor contribution from Late Paleozoic OC to the source of the southern granitoid subarea caused the latter showing distinctly higher εNd(t) than the northern subarea. In conclusion, underplating caused OC remelting which, together with significant magma mixing, produced the East Junggar granitoids in a non-subduction zone environment.

Genesis of post-collisional granitoids and basement nature of the Junggar Terrane, NW China: Nd–Sr isotope and trace element evidence

The post-collisional magmatism of the Junggar Terrane is characterized by intrusion of large amounts of granitoids and minor basic/ultrabasic rocks. The granitoids comprise two magmatic suites: calc-alkaline and alkaline, which were emplaced contemporaneously at about 294 Ma. The calc-alkaline rocks are typically sodium-rich (Na 2O/K 2O=1.1–3.5) and metaluminous (A/NK >1.0, A/CNK=0.78–1.04). They show mildly fractionated REE patterns ((La/Yb) N<15) and spidergrams with strong depletion of Nb, Ti and, to a lesser extent, Sr. The alkaline granites have high contents of SiO 2 (75–78%), alkalis, Nb, HREE, Y, Sn, F and high FeO/MgO ratios and huge Sr and Eu depletion in the spidergrams. Moreover, they display characteristic tetrad REE patterns and non-CHARAC trace element behaviour. The two rock suites have similar initial Nd and Sr isotopic compositions, with ε Nd( T) in the range +5.2 to +7.1 and I Sr mostly in the range 0.7031–0.7041. This points to a predominance of juvenile components in their sources. The calc-alkaline rocks are most probably derived by dehydration-melting of a basic lower crust leaving behind a granulite residue. The process was probably triggered by underplating of mantle-derived basic magmas in an extensional regime. The alkaline granites are considered to have formed by differentiation of the calc-alkaline granitoids. Our study argues for a juvenile continental crust for the basement of the Junggar terrane, which is likely dominated by early Paleozoic oceanic crust and arc complex that were deeply buried during the late Paleozoic subduction and accretion.

Geophysical evidence for an east-dipping Appalachian subduction zone beneath Newfoundland

Geophysical data from within and adjacent to Newfoundland indicate the existence of an undulating but regionally continuous layer of ultramafic rocks generally dipping to the southeast beneath the Avalon zone of Newfoundland. This is interpreted to indicate the southeastward subduction of an early Paleozoic oceanic crust. The ophiolites of western Newfoundland (those both detached and rooted), the granites of north-central Newfoundland, and the Gander ultramafic zone occupy positions that are unique in relation to the ultramafic layer. Northeast of Newfoundland and in New Brunswick, “incomplete” continental suturing has preserved geological zones not seen in Newfoundland.