Shuguang Song | Peking University (original) (raw)

Papers by Shuguang Song

Geochimica et Cosmochimica Acta, 2014

Modern adakite or adakitic rocks are thought to result from partial melting of younger and thus w... more Modern adakite or adakitic rocks are thought to result from partial melting of younger and thus warmer subducting ocean crust in subduction zones, with the melt interacting with or without mantle wedge peridotite during ascent, or from melting of thickened mafic lower crust. Here we show that adakitic (tonalitic-trondhjemitic) melts can also be produced by eclogite decompression during exhumation of subducted and metamorphosed oceanic/continental crust in response to continental collision, as exemplified by the adakitic rocks genetically associated with the early Paleozoic North Qaidam ultra-high pressure metamorphic (UHPM) belt on the northern margin of the Greater Tibetan Plateau. We present field evidence for partial melting of eclogite and its products, including adakitic melt, volumetrically significant plutons evolved from the melt, cumulate rocks precipitated from the melt, and associated granulitic residues. This ``adakitic assemblage'' records a clear progression from eclogite decompression and heating to partial melting, to melt fractionation and ascent/percolation in response to exhumation of the UHPM package. The garnetite and garnet-rich layers in the adakitic assemblage are of cumulate origin from the adakitic melt at high pressure, and accommodate much of the Nb-Ta-Ti. Zircon SHRIMP U-Pb dating shows that partial melting of the eclogite took place at ~435-410 Ma, which postdates the seafloor subduction (>440 Ma) and temporally overlaps the UHPM (~440-425 Ma). While the geological context and the timing of adakite melt formation we observe differ from the prevailing models, our observations and documentations demonstrate that eclogite melting during UHPM exhumation may be important in contributing to crustal growth.

Precambrian Research, 2010

A Paleozoic ultrahigh-pressure metamorphic (UHPM) belt extends along the northern margin of the Q... more A Paleozoic ultrahigh-pressure metamorphic (UHPM) belt extends along the northern margin of the Qaidam Basin, North Tibetan Plateau. Eclogites in the Yuka eclogite terrane, northwest part of this UHPM belt, occur as blocks or layers of varying size intercalated with granitic and pelitic gneisses. These eclogites have protoliths geochemically similar to enriched-type mid-ocean ridge basalts (E-MORB) and oceanic island basalts (OIB). On the basis of Ti/Y ratios, they can be divided into low-Ti and high-Ti groups. The low-Ti group (LTG) eclogites exhibit relatively low TiO2 (most <2.5 wt%) and Ti/Y (<500) but comparatively high Mg# (48–55), whereas the high-Ti group (HTG) eclogites have high TiO2 (most >2.5 wt%) and Ti/Y (>500) but lower Mg# (46–52). Zircons from two eclogite samples gave a magmatic crystallization (protolith) age of ∼850 Ma and a UHPM age of ∼433 Ma. The occurrence, geochemical features and age data of the Yuka eclogites suggest that their protoliths are segments of continental flood basalts (CFBs) with a mantle plume origin, similar to most typical CFBs. Our observation, together with the tectonic history and regional geologic context, lend support for the large scale onset of mantle plume within the Rodinia supercontinent at ∼850 Ma. The Qaidam block is probably one of the fragments of the Rodinia supercontinent with a volcanic-rifted passive margin. The latter may have been dragged to mantle depths by its subducting leading edge of the oceanic lithosphere in the Early Paleozoic.

Journal of Asian Earth Sciences, 2009

Two types of peridotites are recognized in the North Qaidam continental-type UHP metamorphic belt... more Two types of peridotites are recognized in the North Qaidam continental-type UHP metamorphic belt. (1) Garnet peridotite, which includes garnet lherzolite, garnet-bearing dunite, garnet-free dunite and garnet pyroxenite, is one of the most informative lithologies in a continental-type subduction zone. Observations such as diamond inclusion in a zircon crystal and decompression exsolutions in garnet and olivine, plus thermobarometric calculations, argue that this garnet peridotite must have derived from mantle depths in excess of 200 km. Geochemical data reveal that the protolith of the garnet peridotite is largely of cumulate origin from high-Mg melts in a sub-arc mantle wedge environment rather than a abyssal peridotite.

European Journal of Mineralogy, 2009

Coesite, recognized petrographically and confirmed by Raman spectroscopy, is reported from a coar... more Coesite, recognized petrographically and confirmed by Raman spectroscopy, is reported from a coarse-grained eclogite from the Yuka terrane, North Qaidam UHP metamorphic belt. This represents the first record of UHP metamorphism in the western part of this belt. In addition, inclusions of Ab þ Qtz assemblage in garnet with strong evidence for volume expansion may have derived from a Jd þ Coe precursor. Two types of eclogite, coarse-grained and fine-grained, are exposed in the Yuka terrane. The eclogites experienced three distinguishable metamorphic stages: (1) a pre-peak metamorphic stage recorded by the mineral assemblage of Amp þ Pl þ Qtz in the garnet core at P ¼ 0.8-1.0 GPa, T ¼ 450-560 C for the coarse-grained eclogite and P ¼ 1.0-1.2 GPa, T ¼ 450-510 C for the fine-grained eclogite, and by the mineral assemblage of Cpx þ Phn in the garnet mantle at P ¼ 2.25-2.65 GPa, T ¼ 550-610 C for the coarse-grained eclogite; (2) the peak stage characterized by the mineral assemblage of Grt þ Omp þ Phn þ Qtz/Coe þ Rt at P ¼ 3.0 GPa, T ¼ 652 C for the coarsegrained eclogite, and Grt þ Omp þ Phn þ Rt þ Qtz at P ¼ 2.9-3.2 GPa, T ¼ 566-613 C for the fine-grained eclogite; and (3) a retrograde stage recorded by the garnet reaction rims, the breakdown of omphacite, and the pervasive retrograde mineral assemblage of Grt þ Amp þ Pl þ Qtz at P ¼ 0.7-1.1 GPa, T ¼ 550-590 C for the coarse-grained eclogite. The coesite inclusion and the calculated P-T paths suggest that the Yuka UHPM terrane experienced fast and deep subduction ($100 km) under a cool geothermal gradient (6-7 C/km) followed by rapid exhumation.

Comptes Rendus De L Academie Des Sciences Serie Ii Fascicule A-sciences De La Terre Et Des Planetes, 2001

Coesite and graphite were discovered as inclusions in zircon separates from pelitic gneiss associ... more Coesite and graphite were discovered as inclusions in zircon separates from pelitic gneiss associated with a large eclogite body in the North Qaidam UHP terrane. This finding suggests UHP metamorphism at pressures below the diamond stability field. This supports previous indirect UHP evidences, such as polycrystalline quartz inclusions in eclogitic garnet, quartz lamellae in omphacite and P–T estimates for both eclogite and garnet peridotite. The U/Pb and Sm/Nd isotopic ages from the North Qaidam eclogite indicated that continental subduction occurred in Early Palaeozoic, most probably in relation with the collision between the Sino-Korean and Yangtze plates.De la coésite et du graphite ont été identifiés en inclusion dans des zircons de gneiss pélitiques associés à un massif d'éclogite des terrains métamorphiques de très haute pression des chaı̂nons montagneux du Nord du Qaidam, dans le Nord-Est du plateau du Tibet en Chine. Cette découverte suggère un métamorphisme de type UHP dans des conditions de pression inférieures au champ de stabilité du diamant et conforte les observations antérieures de quartz polycristallin en inclusion dans des grenats éclogitiques, de lamelles d'exsolution de quartz dans de l'omphacite et les données P, T préliminaires sur les éclogites et les péridotites. Les âges U/Pb et Sm/Nd des éclogites du Nord-Qaidam indiquent qu'une subduction continentale s'est développée au Paléozoı̈que inférieur, probablement en relation avec la collision des cratons Sino-Koréen et du Yangze.

Lithos, 2010

The Gongshan block near the Eastern Himalayan Syntaxis is a fault-bounded block at the northern t... more The Gongshan block near the Eastern Himalayan Syntaxis is a fault-bounded block at the northern tip of the triangle-shaped Indochina continent (NIC). Exposed in this block are late Paleozoic (Carboniferous to Permian) strata and a north–south belt of intermediate to felsic batholiths (i.e., Gaoligongshan magmatic belt). The contact between the Gaoligongshan batholiths and Carboniferous/Permian strata is characterized by a series of high-grade metamorphic gneisses with leucosome granite veins (i.e., the so-called “Gaoligong Group”). U-Pb SHRIMP and LA-ICP-MS dating of zircons indicate that these gneisses are actually metamorphosed Paleogene sediments containing inherited Archean to Cretaceous detrital zircons (from 2690 to 64 Ma) and have undergone medium- to high-pressure granulite-facies metamorphism at ~ 22 Ma. Leucosome and S-type granite of 22–53 Ma by anatexis are ubiquitous within high-grade metamorphic rocks in the southern part of the Gongshan block. An Early Paleozoic gneissic granite and granitoid intrusions of Jurassic, Cretaceous and Oligocene-Miocene ages are also recognized in NIC blocks. These ages suggest that the NIC differs distinctly from the Indian continent, the Greater and Lesser Himalaya zones, and the Yangtze Craton, but resembles the Lhasa Block in terms of Paleozoic to Mesozoic magmatism and detrital zircon ages. This offers an entirely new perspective on the tectonic evolution of the Gongshan block in particular and of the history of the Lhasa Block in the context of the India-Asia continental collision in general. Furthermore, the high-grade metamorphism in the NIC indicates a strong crustal thickening (vs. strike-slip shearing) event during much of the Eocene to the Oligocene (~ 53–22 Ma) that has brought the Paleogene sediments to depths of greater than 25 km. Continuous northward convergence/compression of the Indian Plate at the Eastern Himalayan Syntaxis may have led to the clockwise rotation, southeastward extrusion and extension of the southeastern part of the Indochina continent.►Gaoligong Group is Paleocene sediments with Archean to Cretaceous detrital zircons. ►The Gongshan block has undergone high-grade metamorphism and anatexis at 53–22 Ma. ►Crustal thickening and rotation was the major teconism before 22 Ma. ►The Indochina continent is the east extension of the Lhasa Block.

Lithos, 2008

Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within gran... more Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol 1 ) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite-eclogite is derived from a troctolitic protolith, whereas the epidote-eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO 2 , and high Al 2 O 3 and MgO. The kyanite-eclogite shows inherited cumulate layering. The phengite-eclogite has high TiO 2 , low Al 2 O 3 and MgO with incompatible trace elements resembling enriched-type MORB. Sr-Nd isotope data indicate that the protoliths of both kyanite-eclogite and epidote-eclogite ([ 87 Sr/ 86 Sr] i~0 .703-0.704; ε Nd (T)~5.9-8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower ε Nd (T) value (1.4-4.1) of phengite-eclogite is more or less consistent with an enriched MORB protolith, their high [ 87 Sr/ 86 Sr] i ratio (0.705-0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P ≥ 2.7 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤ 3.0 GPa. Zircon U-Pb SHRIMP dating shows that the eclogites have a protolith age of 516 ± 8 Ma and a metamorphic age of 445 ± 7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician-Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.

Earth and Planetary Science Letters, 2005

Lithos, 2007

A newly recognized ultrahigh-pressure (UHP) terrane in the Chinese Western Tianshan orogenic belt... more A newly recognized ultrahigh-pressure (UHP) terrane in the Chinese Western Tianshan orogenic belt contains blueschists, eclogites and metapelites. This belt extends westward to the “South Tianshan” in Tajikistan, Kyrgyzstan, Kazakhstan and Uzbekistan for more than 2500 km long in central Asia. New ion microprobe (SHRIMP) U–Pb dating of zircon from HP-UHP eclogites and metapelites indicates Triassic ages for the collision in western Tianshan. Zircon from four eclogites yields magmatic ages of 310–413 Ma in the cores and one metapelite contained detrital zircon cores as old as 1886 ± 20 Ma. Zircon rims reveal the HP-UHP metamorphic ages of 233 ± 4–226 ± 4.6 Ma. The geochronologial data suggest that a South Tianshan paleo-ocean was developed between the Tarim continent and the Yili-central Tianshan Craton before the Carboniferous (> 310 Ma). During the Permian–Triassic subduction and continent collision, oceanic basalts underwent HP/UHP metamorphism. A new tectonic model for HP-UHP metamorphic rocks of the Chinese Western Tianshan orogenic belt represented by HP-UHP metamorphic eclogitic rocks is proposed in the light of recent paleomagnetic, paleontologic, sedimentary and stratigraphic studies.

Two apparently distinct, sub-parallel, paleo-subduction zones can be recognized along the norther... more 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 mélanges 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 (480-440 Ma, with weighted mean ages of 464 AE 6 Ma for North Qilian and 457 AE 7 Ma 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 AE 6 Ma, 40 Myr younger than the age of eclogite formation, and a retrograde age of 403 AE 9 Ma. 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. 423 Ma and exhumed at c. 403 Ma (zircon rim ages in pelitic gneiss).

Lithos, 2011

High-pressure (HP)/low-temperature (LT) metamorphic rocks, such as eclogite and blueschist, are g... more High-pressure (HP)/low-temperature (LT) metamorphic rocks, such as eclogite and blueschist, are generally regarded as an indicator of subduction-zone metamorphism. Eclogites have recently been discovered in the central Qiangtang Block. Their occurrence is highly significant to the understanding of the closure of the Paleo-Tethys and tectonic evolution of northern Tibet. We report the results of petrological, mineralogical and geochronological investigations of these rocks, and discuss their tectonic implications. The Qiangtang eclogite occurs as blocks and lenses in Grt–Phn schist and marble, and is composed of garnet, omphacite, phengite and rutile. Eclogitic garnet contains numerous inclusions, such as glaucophane and phengite in the core, and omphacite in the mantle or inner rim. In strongly retrograded eclogite, the omphacite is replaced by glaucophane, barroisite and albite. Four stages of metamorphic evolution can be determined: (1) prograde blueschist facies; (2) peak eclogite facies; (3) decompression blueschist facies and (4) retrograde greenschist facies. Using the Grt–Omp–Phn geothermobarometer, a peak eclogite facies metamorphic condition of 410–460 °C and 2.0–2.5 GPa was determined. Zircon U–Pb dating gave ages of 230 ± 3 Ma and 237 ± 4 Ma for two eclogite samples. The ages are interpreted as the time of eclogite facies metamorphism. Moreover, 40Ar/39Ar dating of phengite from the eclogite and Grt–Phn schist yielded ages about 220 Ma, which are probably indicative of the time of exhumation to the middle crust. We conclude that the HP/LT metamorphic rocks were formed by northward subduction of the Paleo-Tethys Ocean and they marked a Triassic suture zone between the Gondwana-derived block and Laurasia.► The Qiangtang metamorphic belt lies in the northern Tibetan plateau and consists of blueschist and elcogite. ► Eclogite formed in a relatively low P–T environment. ► U–Pb and Ar–Ar datings show that the eclogite formed in the Triassic. ► Eclogites were formed by northward subduction of the Paleo–Tethys Ocean.

Geochimica Et Cosmochimica Acta, 2009

Fluids released from the subducting oceanic lithosphere are generally accepted to cause mantle we... more Fluids released from the subducting oceanic lithosphere are generally accepted to cause mantle wedge peridotite melting that produces arc magmas. These fluids have long been considered to be dominated by highly oxidized H2O and CO2 as inferred from erupted arc lavas. This inference is also consistent with the geochemistry of peridotite xenoliths in some arc basalts. However, the exact nature of these fluids in the mantle wedge melting region is unknown. Here, we report observations of abundant CH4 + C + H2 fluid inclusions in olivine of a fresh orogenic harzburgite in the Early Paleozoic Qilian suture zone in Northwest China. The petrotectonic association suggests that this harzburgite body represents a remnant of a Paleozoic mantle wedge exhumed subsequently in response to the tectonic collision. The mineralogy, mineral compositions and bulk-rock trace element systematics of the harzburgite corroborate further that the harzburgite represents a high-degree melting residue in a mantle wedge environment. Furthermore, existing and new C, He, Ne and Ar isotopes of these fluid inclusions are consistent with their being of shallow (i.e., crustal vs. deep mantle) origin, likely released from serpentinized peridotites and sediments of the subducting oceanic lithosphere. These observations, if common to subduction systems, provide additional perspectives on mantle wedge melting and subduction-zone magmatism. That is, mantle wedge melting may in some cases be triggered by redox reactions; the highly reduced (∼ΔFMQ–5, i.e., 5 log units below the fayalite–magnetite–quartz oxygen fugacity buffer) CH4-rich fluids released from the subducting slab interact with the relatively oxidized (∼ΔFMQ–1) mantle wedge peridotite, producing H2O and CO2 that then lowers the solidus and incites partial melting for arc magmatism. The significance of slab-component contribution to the geochemistry of arc magmatism would depend on elemental selection and solubility in highly reduced fluids, for which experimental data are needed. We do not advocate the above to be the primary mechanism of arc magmatism, but we do suggest that the observed highly reduced fluids are present in mantle wedge peridotites and their potential roles in arc magmatism need attention.