Constraints on the sources of post-collisional K-rich magmatism: the roles of continental clastic sediments and terrigenous blueschists

Yu Wang, Dejan Prelević, Stephan Buhre, Stephen F. Foley

Research output: Contribution to journalArticleResearchpeer-review

Abstract

The possible role of continental sediments in the generation of potassium-enriched lavas of the Alpine-Himalayan belt depends on their melting behaviour either during subduction or during post-collisional relaxation. Although usually classed as orogenic lavas, these volcanic rocks may result from re-melting of newly formed mantle lithosphere 30-40millionyears after collision ends, and can thus be considered as the first stage of intraplate volcanism. The potassic component in these volcanics is characterized by a high Th/La signature for which there are two competing explanations: melting of subducted continental clastic sediments, and the involvement of lawsonite blueschists in the protoliths to the melting assemblages. Here, we report on a series of high-pressure experiments at 1-3GPa and 900 to 975°C on the melting behaviour of natural phyllite from Serbia, which serves as a proxy for Balkan upper continental crust. Hydrous granitic melts are present in all runs (68wt% SiO2, ~4-5wt% K2O, Mg# <54 and ~5wt% H2O). Garnet, quartz/coesite, plagioclase, K-feldspar, biotite/phengite, clinopyroxene and sillimanite/kyanite, and accessory phases including zircon, rutile, ilmenite, apatite and monazite occur in the charges. LA-ICP-MS analyses establish that the melts are extremely enriched in LILE (except for Sr), Th and U, but depleted in Nb and Zr, with LREE higher than HREE. Accessory phases accommodate several trace elements, especially HFSE and REE. Partition coefficients for some trace elements between residue and crustal melts are close to 1, contrasting strongly with melts of peridotite. Our dataset indicates that the direct melting of upper continental crust alone would generate siliceous, high-K magmas with enriched LILE, Th and U, but cannot explain the high Th/La fingerprint of K-rich lavas of the Alpine-Himalayan orogenic belt. We demonstrate that the Alpine-Himalayan orogenic volcanics attribute their unusual trace element geochemistry to the involvement of lawsonite blueschists that are imbricated together with extremely depleted fore-arc peridotites to form new lithosphere in the source region. There is no need or evidence for deep subduction in which a succession of additional reactions would only serve to modify and dilute the high Th/La signature.

LanguageEnglish
Pages192-207
Number of pages16
JournalChemical Geology
Volume455
DOIs
Publication statusPublished - 20 Apr 2017

Fingerprint

blueschist
clastic sediment
magmatism
Sediments
Melting
melting
Trace Elements
melt
lawsonite
Accessories
trace element
upper crust
continental crust
lithosphere
subduction
Volcanic rocks
Apatites
phyllite
coesite
Quartz

Keywords

  • continental crust recycling
  • intraplate volcanism
  • lawsonite blueschist
  • melting experiments
  • post-collisional K-rich magmatism
  • shallow subduction

Cite this

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title = "Constraints on the sources of post-collisional K-rich magmatism: the roles of continental clastic sediments and terrigenous blueschists",
abstract = "The possible role of continental sediments in the generation of potassium-enriched lavas of the Alpine-Himalayan belt depends on their melting behaviour either during subduction or during post-collisional relaxation. Although usually classed as orogenic lavas, these volcanic rocks may result from re-melting of newly formed mantle lithosphere 30-40millionyears after collision ends, and can thus be considered as the first stage of intraplate volcanism. The potassic component in these volcanics is characterized by a high Th/La signature for which there are two competing explanations: melting of subducted continental clastic sediments, and the involvement of lawsonite blueschists in the protoliths to the melting assemblages. Here, we report on a series of high-pressure experiments at 1-3GPa and 900 to 975°C on the melting behaviour of natural phyllite from Serbia, which serves as a proxy for Balkan upper continental crust. Hydrous granitic melts are present in all runs (68wt{\%} SiO2, ~4-5wt{\%} K2O, Mg# <54 and ~5wt{\%} H2O). Garnet, quartz/coesite, plagioclase, K-feldspar, biotite/phengite, clinopyroxene and sillimanite/kyanite, and accessory phases including zircon, rutile, ilmenite, apatite and monazite occur in the charges. LA-ICP-MS analyses establish that the melts are extremely enriched in LILE (except for Sr), Th and U, but depleted in Nb and Zr, with LREE higher than HREE. Accessory phases accommodate several trace elements, especially HFSE and REE. Partition coefficients for some trace elements between residue and crustal melts are close to 1, contrasting strongly with melts of peridotite. Our dataset indicates that the direct melting of upper continental crust alone would generate siliceous, high-K magmas with enriched LILE, Th and U, but cannot explain the high Th/La fingerprint of K-rich lavas of the Alpine-Himalayan orogenic belt. We demonstrate that the Alpine-Himalayan orogenic volcanics attribute their unusual trace element geochemistry to the involvement of lawsonite blueschists that are imbricated together with extremely depleted fore-arc peridotites to form new lithosphere in the source region. There is no need or evidence for deep subduction in which a succession of additional reactions would only serve to modify and dilute the high Th/La signature.",
keywords = "continental crust recycling, intraplate volcanism, lawsonite blueschist, melting experiments, post-collisional K-rich magmatism, shallow subduction",
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Constraints on the sources of post-collisional K-rich magmatism : the roles of continental clastic sediments and terrigenous blueschists. / Wang, Yu; Prelević, Dejan; Buhre, Stephan; Foley, Stephen F.

In: Chemical Geology, Vol. 455, 20.04.2017, p. 192-207.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Constraints on the sources of post-collisional K-rich magmatism

T2 - Chemical Geology

AU - Wang, Yu

AU - Prelević, Dejan

AU - Buhre, Stephan

AU - Foley, Stephen F.

PY - 2017/4/20

Y1 - 2017/4/20

N2 - The possible role of continental sediments in the generation of potassium-enriched lavas of the Alpine-Himalayan belt depends on their melting behaviour either during subduction or during post-collisional relaxation. Although usually classed as orogenic lavas, these volcanic rocks may result from re-melting of newly formed mantle lithosphere 30-40millionyears after collision ends, and can thus be considered as the first stage of intraplate volcanism. The potassic component in these volcanics is characterized by a high Th/La signature for which there are two competing explanations: melting of subducted continental clastic sediments, and the involvement of lawsonite blueschists in the protoliths to the melting assemblages. Here, we report on a series of high-pressure experiments at 1-3GPa and 900 to 975°C on the melting behaviour of natural phyllite from Serbia, which serves as a proxy for Balkan upper continental crust. Hydrous granitic melts are present in all runs (68wt% SiO2, ~4-5wt% K2O, Mg# <54 and ~5wt% H2O). Garnet, quartz/coesite, plagioclase, K-feldspar, biotite/phengite, clinopyroxene and sillimanite/kyanite, and accessory phases including zircon, rutile, ilmenite, apatite and monazite occur in the charges. LA-ICP-MS analyses establish that the melts are extremely enriched in LILE (except for Sr), Th and U, but depleted in Nb and Zr, with LREE higher than HREE. Accessory phases accommodate several trace elements, especially HFSE and REE. Partition coefficients for some trace elements between residue and crustal melts are close to 1, contrasting strongly with melts of peridotite. Our dataset indicates that the direct melting of upper continental crust alone would generate siliceous, high-K magmas with enriched LILE, Th and U, but cannot explain the high Th/La fingerprint of K-rich lavas of the Alpine-Himalayan orogenic belt. We demonstrate that the Alpine-Himalayan orogenic volcanics attribute their unusual trace element geochemistry to the involvement of lawsonite blueschists that are imbricated together with extremely depleted fore-arc peridotites to form new lithosphere in the source region. There is no need or evidence for deep subduction in which a succession of additional reactions would only serve to modify and dilute the high Th/La signature.

AB - The possible role of continental sediments in the generation of potassium-enriched lavas of the Alpine-Himalayan belt depends on their melting behaviour either during subduction or during post-collisional relaxation. Although usually classed as orogenic lavas, these volcanic rocks may result from re-melting of newly formed mantle lithosphere 30-40millionyears after collision ends, and can thus be considered as the first stage of intraplate volcanism. The potassic component in these volcanics is characterized by a high Th/La signature for which there are two competing explanations: melting of subducted continental clastic sediments, and the involvement of lawsonite blueschists in the protoliths to the melting assemblages. Here, we report on a series of high-pressure experiments at 1-3GPa and 900 to 975°C on the melting behaviour of natural phyllite from Serbia, which serves as a proxy for Balkan upper continental crust. Hydrous granitic melts are present in all runs (68wt% SiO2, ~4-5wt% K2O, Mg# <54 and ~5wt% H2O). Garnet, quartz/coesite, plagioclase, K-feldspar, biotite/phengite, clinopyroxene and sillimanite/kyanite, and accessory phases including zircon, rutile, ilmenite, apatite and monazite occur in the charges. LA-ICP-MS analyses establish that the melts are extremely enriched in LILE (except for Sr), Th and U, but depleted in Nb and Zr, with LREE higher than HREE. Accessory phases accommodate several trace elements, especially HFSE and REE. Partition coefficients for some trace elements between residue and crustal melts are close to 1, contrasting strongly with melts of peridotite. Our dataset indicates that the direct melting of upper continental crust alone would generate siliceous, high-K magmas with enriched LILE, Th and U, but cannot explain the high Th/La fingerprint of K-rich lavas of the Alpine-Himalayan orogenic belt. We demonstrate that the Alpine-Himalayan orogenic volcanics attribute their unusual trace element geochemistry to the involvement of lawsonite blueschists that are imbricated together with extremely depleted fore-arc peridotites to form new lithosphere in the source region. There is no need or evidence for deep subduction in which a succession of additional reactions would only serve to modify and dilute the high Th/La signature.

KW - continental crust recycling

KW - intraplate volcanism

KW - lawsonite blueschist

KW - melting experiments

KW - post-collisional K-rich magmatism

KW - shallow subduction

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U2 - 10.1016/j.chemgeo.2016.10.006

DO - 10.1016/j.chemgeo.2016.10.006

M3 - Article

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EP - 207

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

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