North Qilian Suture Zone Research Articles

Petrogenesis of Syn-Collisional Adakitic Granitoids and Their Copper Mineralization Potential in the North Qilian Suture Zone

The petrogenesis of late Ordovician–early Silurian adakitic plutons in the North Qilian suture zone (NQSZ) and their copper mineralization potential remain poorly understood. Here we present a detailed study of the Heishishan (HSS) granodiorite–granite pluton, spatially associated with Cu–Au mineralization in the eastern section of the NQSZ. Zircon U–Pb dating confirms that the granodiorite–granite were formed at ca. 438–435 Ma, in association with a continental collision. Geochemically, the granitoids resemble low-Mg adakitic rocks featured by elevated Sr/Y and (La/Yb)N ratios with depleted MgO, Cr, and Ni concentrations, suggesting minimal mantle contribution. They are sodium rich with K2O/Na2O < 1, and have higher and more varied Sr/Y, but lower La/Yb than those from the continental lower crust. The εHf(t) values of zircon grains are positive and vary in a wide range of +2.0–12.7, indicating a heterogeneous source rather than a single arc basaltic source. They show moderately radiogenic Sr and Nd isotope compositions with initial 87Sr/86Sr ratios of 0.705101–0.706312 and εNd(t) values of +0.5–1.0, most likely a mixed source of the oceanic basaltic crust plus ca. 15–20% overlying sediments. The magmatic oxygen fugacity was relatively low as indicated by zircon Ce(IV)/Ce(III) ratios of 32–156, which is unfavorable for a large copper mineralization.

Sediment recycling and adakite petrogenesis: Constraints from the late Ordovician tonalite in the North Qilian suture zone

Sediment recycling and adakite petrogenesis: Constraints from the late Ordovician tonalite in the North Qilian suture zone

Detrital zircons from high-pressure trench sediments (Qilian Orogen): Constraints on continental-arc accretion, subduction initiation and polarity of the Proto-Tethys Ocean

The Qilian Orogen in the northern Tibetan plateau records the tectonic history of the Proto-Tethys Ocean from its initial spreading, subduction to final closure. However, the timing of subduction initiation, subduction polarity and arc volcanic processes of the Proto-Tethys Ocean remain controversial. In this contribution, we present geochronological and Hf isotopic studies for 812 detrital zircons from blueschist- to eclogite-facies metamorphic sedimentary rocks from the trench and accretionary wedge in the Proto-Tethys subduction zone in the North Qilian suture zone. Out of all the U-Pb age data, the Neoproterozoic to Early Paleozoic zircons reveal the timing of opening and closure of the Proto-Tethys Ocean. We show that ∼50 % of the original sediments were derived from the arc-related magmas, with ages ranging from 535 Ma to 441 Ma. This range represents the time from subduction initiation to closure of the Proto-Tethys Ocean in the Qilian Orogen. Based on the closer resemblance of statistical ages to the crust to the north (Alxa) than the south (Central Qilian), we believe the initial subduction polarity of the Proto-Tethys Ocean is northward. The corresponding εHf(t) values (−11 ∼ +15) shows characteristics of continental arc formed by subduction process. The massive intrusion of arc magma during subduction led to a significant crustal growth, and the crustal thickness estimates based on Eu anomalies indicate that the thickness of the continental margin varies from ∼60 km at 500–490 Ma to ∼45 km at ca. 450 Ma, suggesting an evolution process from compression to back-arc extension.

Episodic Melt Percolating and Chromite Mineralization in Yushigou Peridotite, North Qilian Suture Zone

AbstractHarzburgite‐dunite associations are the dominant type of ultramafic heterogeneities in the exhumed peridotites. Although the dunite network are usually small in volume of exposed mantle rocks, their mineral structures and chemical variations are widely observed in various tectonic environments and record complex mantle evolution. The Yushigou peridotite complex displays a 25 km2 mantle section belonging to a partly dismembered ophiolite in North Qilian suture zone, northwest China. It is mostly composed of depleted spinel harzburgite re‐equilibrated at temperatures of 900 °C, contain dunite veins and minor podiform chromite at its Moho level.Here we report the petrological observations and mineral chemical analyses of harzburgite‐dunite associations in the Yushigou peridotite complex. Based on the data we classified high Cr # (51–74) and low Cr# (19–34) dunite veins in this Yushigou complex. The podiform chromite are mainly high Cr# affinity and are production of melt‐harzburgite interaction. There are three main microstructures recorded the episodic melt‐rock interaction process: (1) complex spinel mineral inclusions both in harzburgite and two kind of dunite record widespread melt percolating. The observed primary inclusions (not related to later low temperature fluid activities) including hydrous and anhydrous silicates and minor sulphides, such as PGE and Fe‐Cu‐Ni related sulfidies, olivine, orthopyroxene, clinopyroxene, amphibole and phlogopite. (2) In all dunite samples, clinopyroxene dominated sieve‐textured rims around spinel are more abundant and more thicken than the harzburgite; implying stained/pulsated melt supply after the chromite crystallization. (3) Millimeter scale veined reaction patches in some dunite samples consist of clinopyroxene, amphibole, phlogopite, Fe‐Ni sulfidies and carbonates. Such metasomatic minerals assemblages may response for the later fluid infiltration after chromite mineralization. All these petrological observations and mineral chemical compositions (major and trace element contents) thus provide a wealth of information on the pathways of migrating melts in the mantle wedge, and provide constrain on nature and evolution of the uprising magma. Strong enrichment of incompatible trace elements in dunite clinopyroxene (LREE/HREE >10) possible reflect a slab origin of the percolating melt which is not observed elsewhere participating in chromite forming.

Late Cambrian tonalite-trondhjemite association in the eastern segment of North Qilian suture zone: petrogenesis and geodynamic implications

ABSTRACT The late Cambrian tonalite-trondhjemite association in the eastern segment of North Qilian suture zone (E. NQSZ) is important for understanding of the sub-arc magmatic process and tectonic evolution. Here we report data for feldspar-quartz porphyry and fine-grained trondhjemite with abundant tonalite enclave/interlayer from the Leigongshan (LGS) pluton, and medium-grained tonalite from the Beilinggou (BLG) pluton. Zircon U‒Pb ages suggest emplacement of the plutons at ca. 506‒481 Ma, associated with early subduction of the North Qilian Ocean. The LGS tonalite enclave/interlayers and trondhjemite hosts share many characteristics in common, including the same mineralogy and texture, similar mineral chemistry, whole-rock trace element ratio and Sr‒Nd isotopic compositions. The tonalite enclave/interlayers probably represent quenched blobs/layers of an evolved, tonalitic magma branch resulting from periodic injection of magma into the magma chamber. All the LGS-BLG tonalite-trondhjemites display low abundances of incompatible trace elements (e.g., K, Rb, Th, Zr and REE) and depleted Sr‒Nd‒Hf isotopes (87Sr/86Sr = 0.707490‒0.707840; εNd(t) = +4.0‒4.3; zircon εHf(t) = +4.0‒13.1), indicating little/no contribution of the continental crust. Geochronological and geochemical consistency between the coeval arc sodium-rich volcanic rocks and studied tonalite-trondhjemites further suggests crystal fractionation of a common basaltic parental magma. The parental magma was generated in continental-arc rifting environments due to slab rollback in the early stage of subduction.

Multiple subduction processes of the Proto-Tethyan Ocean: Implication from Cambrian intrusions along the North Qilian suture zone

Cambrian mafic to felsic intrusive rocks intruding Precambrian rocks are recognized along the southern margin of the North Qilian suture zone. These intrusive rocks predate final closure of the North Qilian Ocean and consideration of these rocks is essential to any reconstruction of the tectonic framework of the Qilian Orogen. They include 517 Ma transitional IS type granite, 510 Ma I-type calc-alkaline diorite, 509–505 Ma calc-alkaline gabbro, and 498 Ma shoshonitic monzonite as well as highly fractionated syenogranite. Distinctive geochemical signatures and Sr-Nd-Hf isotopic compositions indicate derivation from multiple magma sources associated with subduction. Moreover, tectonostratigraphic correlation indicates that the Precambrian rocks along the southern margin of the North Qilian suture zone were associated with the Central Qilian microcontinent. These Cambrian mafic to felsic intrusive rocks are likely to be magmatic products of a continental arc built on the northern margin of the Central Qilian microcontinent during the processes of southward subduction of the North Qilian Ocean. Together, a Cambrian Andean-type arc, Cambrian-Ordovician Mariana-type intra-oceanic arc and multiple Early Paleozoic accretionary complexes within the North Qilian suture zone attest to various subduction-accretion processes that occurred within the Proto-Tethyan Ocean from Cambrian to Middle Ordovician time prior to terminal continental collision.

Petrogenesis and tectonic implications of cambrian Nb-enriched I- and aluminous A-type granites in the North Qilian suture zone

ABSTRACT Granitoids in active continental margins are probes of sub-arc magmatic processes and are crucial for the understanding of crust–mantle interaction during subduction. Here we report data for the middle to late Cambrian Tuole (TL) and Dabaishitou (DBST) granitic plutons in the western segment of the North Qilian suture zone (NQSZ). The TL-DBST plutons comprise monzonite, alkali-feldspar granite, gabbroic diorite, monzonitic porphyry and biotite monzogranite. Zircon U‒Pb ages reveal that these granitoids crystallized at ca. 509‒492 Ma, consistent with the early subduction of the North Qilian Ocean. Mineralogical, petrological, and geochemical features indicate that the TL monzonite and gabbroic diorite are both high-K calc-alkaline I-type granitoids but the latter is Nb-enriched; the TL alkali-feldspar granites and DBST granitoids belong to aluminous A-type and transitional I- and A-type granites, respectively. The I- and aluminous A-type granites show similar and comparable isotopic compositions, e.g., εHf(t) values of ‒5.6 to +2.7 versus ‒2.4 to +3.5, but different melting conditions, e.g., high temperature and low oxygen fugacity for aluminous A-type granites as indicated by zircon trace elements. Geochemical and isotope data suggest that all these granitoids were mainly derived from the sub-arc mantle metasomatized by slab-derived fluids/asthenospheric mantle-derived melts/both prior to melting with minor input of LCC-derived melts (ca. 10–20%). The HFSE enrichment feature was inherited by granites and further enhanced by protracted crystal fractionation at low oxygen fugacity conditions to produce the TL aluminous A1-type granites but weakened by fractionation at relatively high oxygen fugacity conditions to produce the DBST I-A-transitional-type granite. Partial melting of the sub-arc lithospheric mantle was induced by upwelling of the asthenospheric mantle and their interactions in response to slab rollback during subduction initiation (ca. 509–506 Ma), and by addition of slab-derived fluids again as subduction proceeded (ca. 493–492 Ma).

Plastic Deformation and Seismic Properties in Fore-arc Mantles: A Petrofabric Analysis of the Yushigou Harzburgites, North Qilian Suture Zone, NW China

Plastic Deformation and Seismic Properties in Fore-arc Mantles: A Petrofabric Analysis of the Yushigou Harzburgites, North Qilian Suture Zone, NW China

Microstructures and petro-fabrics of lawsonite blueschist in the North Qilian suture zone, NW China: Implications for seismic anisotropy of subducting oceanic crust

We conducted a detailed study on the microstructures and petro-fabrics of massive and foliated lawsonite blueschist (LBS) in North Qilian suture zone, NW China. The lattice preferred orientation (LPO) of glaucophane and lawsonite in foliated lawsonite blueschist (LBS) is considered to be dominantly formed by the deformation mechanism of dislocation creep and rigid-body rotation, respectively. The LPO of glaucophane is mainly characterized by the [001] axis aligning parallel to lineation and the [100] axis and (110) pole plunging perpendicular to foliation. In contrast, the LPO of lawsonite features the maximum [010] axis concentrated close to lineation and the [001] axis strongly clustered normal to foliation. The preferred orientation of [010] axis of lawsonite parallel to lineation is supported by a two-dimensional numerical modeling using the finite-volume method (FVM). The mineral LPOs are much stronger in foliated LBS than in massive LBS. In addition, a kinematic vorticity analysis suggests that both pure shear dominant (Wm=0.18–0.26) and simple shear dominant (Wm=0.86–0.93) deformation regimes are present in foliated LBS. The [001] axis and (010) pole of glaucophane, and the [100] and [010] axes of lawsonite, tend to distribute in a foliation–parallel girdle in the pure shear dominant samples, but simple shear dominant samples display more lineation–parallel concentrations of a [001] axis of glaucophane and a [010] axis of lawsonite. Because the whole-rock seismic anisotropies in foliated LBS are significantly higher than those in massive LBS and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite, the delay time of fast S-wave polarization anisotropy induced by an actual subducting oceanic crust with a high subducting angle (>45–60°) is expected to range from 0.03 to 0.09s (lower bound for massive LBS) and from 0.1 to 0.3s (upper bound for foliated epidote blueschist).

Melting of continental crust during subduction initiation: A case study from the Chaidanuo peraluminous granite in the North Qilian suture zone

The Chaidanuo granite batholith is a ∼500km2 homogeneous, high-level intrusion in the North Qilian oceanic suture zone. Three types of enclaves have been recognized, including (1) supracrustal xenoliths, (2) biotite gneiss that represents restite of the upper crustal melting, and (3) coeval mantle-derived mafic magmatic enclave (MME). The batholith consists dominantly of peraluminous biotite monzogranite with SiO2 69–73wt.% and A/CNK 1.05–1.28 and shows geochemical affinity with the upper-continental crust, e.g., enrichment of large ion lithophile elements (LILEs; K, Rb, U, Th, Pb), depletion of high field strength elements (HFSEs; Nb, Ta, Ti), P, Eu, and Sr, and enriched Sr ([87Sr/86Sr]i, 0.731 to 0.744)–Nd (εNd (t), −6.0 to −7.1) isotopes. Zircon U–Pb dating indicates that this batholith formed at 516–505Ma, coeval with the MMEs (510Ma), which represent the early stage of seafloor subduction in the North Qilian suture zone. The Hf isotopic composition of the MME (εHf (t)=−6.0 to +2.1) is more depleted than that of the host granite (εHf (t)=−14.8 to −4.7), suggesting mixing of mantle- and crust-derived melts. The inherited zircon cores in the host granite yield an age peak at ∼750Ma with a few detritals of 591–1683Ma. Zircons from a biotite gneiss enclave yield a protolith age of 744Ma and a metamorphic/melting event at 503Ma. Sr–Nd isotopic modeling reveals that the batholith was generated by melting of Neoproterozoic granitic rocks with the addition of 10–18% mantle-derived magma. Therefore, peraluminous granite can be produced by melting of crustal materials heated by mantle-derived magmas during the early stage of subduction initiation at the site of a prior passive continental margin.

Petro‐fabrics and seismic properties of blueschist and eclogite in the North Qilian suture zone, NW China: Implications for the low‐velocity upper layer in subducting slab, trench‐parallel seismic anisotropy, and eclogite detectability in the subduction zone

AbstractThe potential seismological contributions of metamorphosed and deformed oceanic crust in a subduction zone environment were studied in a detailed petro‐fabric analysis of blueschist and eclogite in the North Qilian suture zone, NW China. The calculated whole‐rock seismic properties based on the measured lattice preferred orientations of the constituting minerals show increasing P‐wave and S‐wave velocities and decreasing seismic anisotropies from blueschist to eclogite, mainly due to the decreasing volume proportion and deformation extent of glaucophane. The low velocity of the upper layer in the subducting oceanic crust can be explained by the existence of blueschist and foliated eclogite, which induces a 3–12% reduction in velocity compared to that induced by the surrounding mantle rocks. This low‐velocity layer may gradually disappear when blueschist and foliated eclogite are replaced by massive eclogite at a depth in excess of 60–75 km for the paleo North Qilian subduction zone. Trench‐parallel seismic anisotropy with a moderate delay time (0.1–0.3 s) can only effectively contribute to deformed blueschist and eclogite in a high‐angle (>45–60°) subducting slab, regardless of the direction of slab movement. The calculated reflection coefficients (Rc = 0.04–0.20) at the lithologic interfaces between eclogite and blueschist imply that it may be possible to detect eclogite bodies in shallow subduction channels using high‐resolution seismic reflection profiles. However, the imaging of eclogite bodies located in deep subduction zones could be challenging.

Tholeiite–Boninite terrane in the North Qilian suture zone: Implications for subduction initiation and back-arc basin development

A tholeiite–boninite terrane occurs as a ~4.5-km-thick massif with lavas and intrusions in the Dachadaban (DCDB) area, the middle part of the North Qilian oceanic-type suture zone. It comprises two distinct lithological groups: the lower tholeiite unit and the upper boninite unit. The lower tholeiite unit consists of massive lava flows and subordinate gabbro intrusions with MORB-like characteristics that could represent 5–6% melting of an enriched MORB mantle. In contrast, the overlying boninite unit consists of pillow lavas, dolerite dykes and gabbro intrusions and shows high-Ca boninite features that may be formed by continuous melting of the extremely refractory mantle with the aid of a combination of the elevated mantle potential temperature of 1380–1460°C at depths of 42–66km and involvement of slab-derived hydrous fluids/melts. Zircon U–Pb SHRIMP dating shows that lower tholeiite magmatism lasted for at least 12M.y. from 517Ma to 505Ma and upper boninite volcanism occurred between 505 and 487Ma, which is consistent with the earliest age (486±7Ma) of the SSZ-type ophiolite belt immediately north of the Dachaidaban (DCDB) tholeiite–boninite terrane. The lower tholeiites are considered to represent the products of earliest infant arc magmatism by decompression-induced partial melting of the relatively “dry” and fertile upwelling mantle in response to the onset of subduction. The upper boninite unit with younger age of 487±9Ma is interpreted as earliest products of infant arc splitting and subsequent back-arc basin development. Therefore, the long-lived DCDB tholeiite–boninite sequence presents a key lithological record of early stages of supra-subduction zone magmatic activity evolved from subduction initiation at ~517Ma to back-arc extension at ~487Ma.

Variation of mineral composition, fabric and oxygen fugacity from massive to foliated eclogites during exhumation of subducted ocean crust in the North Qilian suture zone, NW China

Eclogites from the North Qilian suture zone are high-pressure low-temperature metamorphic rocks of ocean crust protolith, and occur in both massive and foliated varieties as individual blocks of tens to hundreds of metres in size. The massive type is weakly deformed and shows granoblastic texture characterized by a coarse-grained peak mineral assemblage of Grt1 + Omp1 + Ph + Rt ± Lws (or retrograde Cz). In contrast, the foliated type is strongly deformed and shows a fine-grained retrograde mineral assemblage of Grt2 + Omp2 + Cz + Gln + Ph. Both total FeO and aegirine contents in omphacite, as well as XFe[=Fe3+/(Fe3+ + AlVI)] in clinozoisite/epidote, increase significantly from massive to foliated eclogites. Lattice preferred orientation (LPO) of omphacite, determined by electron back-scatter diffraction analysis, is characterized by weak and strong SL-type fabrics for massive and foliated eclogites, respectively. Clinozoisite/epidote also developed SL-type fabric, but different from the LPOs of omphacite in and axes, owing to their opposite crystallographic long and short axis definitions. The transition of deformation mechanism from dislocation creep to diffusive mass transfer (DMT) creep in omphacite and the concomitant retrograde metamorphism both are efficiently facilitated when the original coarse-grained Omp1 + Grt1 + Lws assemblage is dynamically recrystallized and retrogressed into the fine-grained Fe3+-rich assemblage of Omp2 + Grt2 + Cz + Gln. The DMT process with concomitant anisotropic growth assisted by fluids is considered to be an important deformation mechanism for most minerals in the foliated eclogite. P–T estimates yielded 2.3–2.6 GPa and 485−510 °C for the massive eclogite and 1.8–2.2 GPa and 450−480 °C for the foliated eclogite. The significant increase in total Fe and Fe3+ contents in omphacite and clinozoisite/epidote from massive to foliated eclogite suggests changes in mineral compositions accompanied by an increase in oxygen fugacity during ductile deformation associated with exhumation. The LPO transition of omphacite, clinozoisite and rutile from weak SL-type in massive eclogites to strong SL-type in foliated eclogites is interpreted to represent the increment of shear strain during exhumation along the ‘subduction channel’.

Eclogite and carpholite‐bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle

AbstractLow‐temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high‐pressure low‐temperature (HPLT) subduction complex (mélange) in the North Qilian oceanic‐type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid‐bearing schists. Mg‐carpholite was identified in some high‐Mg chloritoid schists.P–Testimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg‐carpholite schist. Mineral assemblages andP–Testimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi‐pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred atc.454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peakP–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction‐zone gradients (6–7 °C km−1), and this cold subduction brought a large amount of H2O to the deep mantle in the Early Palaeozoic times.

Evolution from Oceanic Subduction to Continental Collision: a Case Study from the Northern Tibetan Plateau Based on Geochemical and Geochronological Data

Two apparently distinct, sub-parallel, paleo-subduction zones can be recognized along the northern margin of the Tibetan Plateau: the North Qilian Suture Zone (oceanic-type) with ophiolitic melanges and high-pressure eclogites and blueschists in the north, and the North Qaidam Belt (continental-type) in the south, an ultrahighpressure (UHP) metamorphic terrane comprising pelitic and granitic gneisses, eclogites and garnet peridotites. Eclogites from both belts have protoliths broadly similar to mid-ocean ridge basalts (MORB) or oceanic island basalts (OIB) in composition with overlapping metamorphic ages (480–440Ma, with weighted mean ages of 464 6Ma for North Qilian and 457 7Ma for North Qaidam), determined by zircon U–Pb sensitive high-resolution ion microprobe dating. Coesite-bearing zircon grains in pelitic gneisses from the North Qaidam UHP Belt yield a peak metamorphic age of 423 6Ma, 40Myr younger than the age of eclogite formation, and a retrograde age of 403 9Ma. These data, combined with regional relationships, allow us to infer that these two parallel belts may represent an evolutionary sequence from oceanic subduction to continental collision, and continental underthrusting, to final exhumation. The Qilian–Qaidam Craton was probably a fragment of the Rodinia supercontinent with a passive margin and extended oceanic lithosphere in the north, which was subducted beneath the North China Craton to depths >100 km at c. 423Ma and exhumed at c. 403Ma (zircon rim ages in pelitic gneiss).