Subduction erosion and crustal material recycling indicated by adakites in central Tibet

Subduction erosion is important for crustal material recycling and is widespread in modern active convergent margins. However, such a process is rarely identified in fossil convergent systems, which casts doubt on the importance of subduction erosion through the geological record. We report on ca. 155 Ma Kangqiong (pluton) intrusive rocks of a Mesozoic magmatic arc in the southern Qiangtang terrane, central Tibet. These rocks mainly consist of trondhjemites and tonalites and are similar to slab-derived adakites with mantle-like zircon oxygen isotope compositions (δ18O = 5.2‰–5.6‰), they display more evolved Sr-Nd isotopes and higher Th/La relative to mid-oceanic ridge basalts from the Bangong-Nujiang suture, and they contain abundant amphibole and biotite. These characteristics indicate magma generation via H2O-fluxed melting of eroded forearc crust debris with subducted oceanic crust at 1.5–2.5 GPa and 700–800 °C. In addition, the intrusions are exposed <20 km north of the Bangong-Nujiang suture. Given the formation of adakites, narrow arc-suture distance, migration of the Jurassic frontal arc toward the continent interior, and other independent geological archives, we suggest that the hydrated forearc crust materials were removed from the overlying plate and carried into the mantle by subduction erosion. Our study provides the first direct magmatic evidence for a subduction erosion process in pre-Cenozoic convergent systems, which confirms an important role for such processes in subduction-zone material recycling.

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Supplemental Material: Subduction erosion and crustal material recycling indicated by adakites in central Tibet

10.1130/geol.s.13708522.v1 ◽

2021 ◽

Author(s):

Zong-Yong Yang ◽

QIANG WANG ◽

et al.

Keyword(s):

Analytical Methods ◽

Crustal Material ◽

Material Recycling ◽

Subduction Erosion ◽

Central Tibet

Supplemental figures, analytical methods and results, and data and results tables.<br>

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Supplemental Material: Crustal material recycling induced by subduction erosion and subduction-channel exhumation: A case study of central Tibet (western China) based on P-T-t paths of the eclogite-bearing Baqing metamorphic complex

10.1130/gsab.s.13190657 ◽

2020 ◽

Author(s):

Xin Jin ◽

Yu-Xiu Zhang ◽

Kai-Jun Zhang ◽

et al.

Keyword(s):

Western China ◽

Metamorphic Rocks ◽

Crustal Material ◽

Metamorphic Complex ◽

Subduction Channel ◽

Material Recycling ◽

Subduction Erosion ◽

Mineral Compositions ◽

Central Tibet

Compositional mapping images of one garnet, Triassic paleo-geographic facies of Qiangtang, summarized published Paleozoic and Proterozoic ages in Tibetan Plateau and Himalaya, mineral compositions, and chronology data of the Baqing metamorphic rocks.

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Supplemental Material: Subduction erosion and crustal material recycling indicated by adakites in central Tibet

10.1130/geol.s.13708522 ◽

2021 ◽

Author(s):

Zong-Yong Yang ◽

QIANG WANG ◽

et al.

Keyword(s):

Analytical Methods ◽

Crustal Material ◽

Material Recycling ◽

Subduction Erosion ◽

Central Tibet

Supplemental figures, analytical methods and results, and data and results tables.<br>

Download Full-text

Supplemental Material: Crustal material recycling induced by subduction erosion and subduction-channel exhumation: A case study of central Tibet (western China) based on P-T-t paths of the eclogite-bearing Baqing metamorphic complex

10.1130/gsab.s.13190657.v1 ◽

2020 ◽

Author(s):

Xin Jin ◽

Yu-Xiu Zhang ◽

Kai-Jun Zhang ◽

et al.

Keyword(s):

Western China ◽

Metamorphic Rocks ◽

Crustal Material ◽

Metamorphic Complex ◽

Subduction Channel ◽

Material Recycling ◽

Subduction Erosion ◽

Mineral Compositions ◽

Central Tibet

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Late Jurassic sodium-rich adakitic intrusive rocks in the southern Qiangtang terrane, central Tibet, and their implications for the Bangong–Nujiang Ocean subduction

Lithos ◽

10.1016/j.lithos.2015.10.014 ◽

2016 ◽

Vol 245 ◽

pp. 34-46 ◽

Cited By ~ 28

Author(s):

Yalin Li ◽

Juan He ◽

Zhongpeng Han ◽

Chengshan Wang ◽

Pengfei Ma ◽

...

Keyword(s):

Late Jurassic ◽

Intrusive Rocks ◽

Central Tibet ◽

Qiangtang Terrane

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O–H–Sr–Nd isotope constraints on the origin of the Famatinian magmatic arc, NW Argentina

Geological Magazine ◽

10.1017/s0016756820000321 ◽

2020 ◽

Vol 157 (12) ◽

pp. 2067-2080 ◽

Cited By ~ 2

Author(s):

P. Alasino ◽

C. Casquet ◽

C. Galindo ◽

R. Pankhurst ◽

C. Rapela ◽

...

Keyword(s):

Metamorphic Rocks ◽

Crustal Material ◽

Nd Isotopes ◽

Nd Isotope ◽

Sierras Pampeanas ◽

Mafic Rocks ◽

Oxygen Isotope Exchange ◽

Magmatic Arc ◽

Zircon Crystallization ◽

Major Process

AbstractWe report a study of whole-rock O–H–Sr–Nd isotopes of Ordovician igneous and metamorphic rocks exposed at different crustal palaeodepths along c. 750 km in the Sierras Pampeanas, NW Argentina. The isotope compositions preserved in the intermediate rocks (mostly tonalite) (average δ18O = +8.7 ± 0.5‰, δD = −73 ± 14‰, 87Sr/86Srt = 0.7088 ± 0.0001 and εNdt = −4.5 ± 0.6) show no major difference from those of most of the mafic rocks (average δ18O = +8 ± 0.8‰, δD = −84 ± 18‰, 87Sr/86Srt = 0.7082 ± 0.0016 and εNdt = −4 ± 1.1), suggesting that most of their magmas acquired their crustal characteristics in the mantle. The estimate of assimilation of crustal material (δ18O = +12.2 ± 1.7‰, δD = −89 ± 21‰, 87Sr/86Srt = 0.7146 ± 0.0034 and εNdt = −6.9 ± 0.7) by the tonalite is in most samples within the range 10–20%. Felsic magmas that reached upper crustal levels had isotope values (δ18O = +9.9 ± 1.5‰, δD= −76 ± 5‰, 87Sr/86Srt = 0.7067 ± 0.0010, εNdt = −3.5 ± 1.4) suggesting that they were not derived by fractionation of the contaminated intermediate magmas, but evolved from different magma batches. Some rocks of the arc, both igneous (mostly gabbro and tonalite) and metamorphic, underwent restricted interaction with meteoric fluids. Reported values of δ18O of magmatic zircons from the Famatinian arc rocks (+6 to +9‰) are comparable to our δ18O whole-rock data, indicating that pervasive oxygen isotope exchange in the lower crust was not a major process after zircon crystallization.

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Crustal material recycling induced by subduction erosion and subduction-channel exhumation: A case study of central Tibet (western China) based on P-T-t paths of the eclogite-bearing Baqing metamorphic complex

Geological Society of America Bulletin ◽

10.1130/b35638.1 ◽

2020 ◽

Author(s):

Xin Jin ◽

Yu-Xiu Zhang ◽

Donna L. Whitney ◽

Kai-Jun Zhang ◽

Natalie H. Raia ◽

...

Keyword(s):

Mantle Wedge ◽

Crustal Material ◽

Metamorphic Complex ◽

The South ◽

Subduction Channel ◽

Crustal Recycling ◽

The North ◽

Subduction Erosion ◽

Proterozoic Basement ◽

Central Tibet

Subduction and exhumation processes, interacting with each other, play a key role in crustal recycling. Downgoing oceanic lithosphere constitutes the dominant input at subduction margins, but subduction erosion, the removal of crustal material from the overriding plate, may add additional ingredients and complexity to the subduction factory. Different exhumation models have been proposed to explain how subducted materials are exhumed and therefore contribute to crustal recycling, e.g., exhumation up the subduction channel versus diapiric rise through the mantle wedge that overlies the subducted plate. The recently discovered Baqing eclogite-bearing high-pressure metamorphic complex, central Tibet, China, provides an excellent opportunity to decode the exhumation process, the origin of subduction-related magmatism, and the crustal structure of the North Qiangtang block, in addition to elucidating processes of crustal recycling. Pressure-temperature-time (P-T-t) paths and zircon U-Pb ages and trace-element compositions for Baqing high-pressure rocks were used to evaluate exhumation processes and to determine the geochemical and tectonic affinity of the Baqing metamorphic complex. The Baqing metamorphic complex is mainly composed of eclogite, gneiss, and schist. It is located between two geologically distinct terranes—the South Qiangtang block, which has early Paleozoic basement, and the North Qiangtang block, which has Proterozoic basement. In the schist, zircon cores with steep heavy rare earth element (HREE) slopes and oscillatory zoning yielded inherited ages that are similar to detrital zircon ages for the South Qiangtang block schist; in contrast, zircon rims with flat HREE slopes yielded metamorphic ages of 224 Ma that are similar to the metamorphic ages obtained for the Baqing eclogite. In contrast, zircons from the gneiss yielded an upper-intercept age of 1033 ± 32 Ma (interpreted as the crystallization age) and a lower-intercept metamorphic age of 198 ± 4 Ma. Field relations indicate that gneiss and eclogite/amphibolite were exhumed together, so the ∼20 m.y. gap between the gneiss and the metabasite metamorphism may indicate a long exhumation duration. In the region, Proterozoic ages of ca. 1000 Ma are known only from the North Qiangtang block; we thus propose that the Baqing gneiss originated from North Qiangtang block Proterozoic basement, which, along with North Qiangtang block Triassic arc magmatic rocks and the discrepancies between ancient and current arc-trench distances, results in estimates of ∼20−170 km of Triassic subduction erosion. Results of P-T analyses show that most eclogite, amphibolite, and schist shared a similar clockwise P-T path, different from that of the gneiss, which records a higher geothermal gradient. The clockwise P-T trajectory, long exhumation duration, lack of significant heating during exhumation, and the South Qiangtang block affinity of the schist (host rock of the Baqing eclogite) are consistent with subduction-channel exhumation rather than diapiric rise through the mantle wedge. Geochemical similarities between the North Qiangtang block Triassic subduction-related rocks and the Baqing gneiss may signal the involvement of unexhumed Baqing metamorphic complex in the recycling of the Qiangtang crust.

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