Magmatic peridotites and pyroxenites, Andong Ultramafic Complex, Korea: Geochemical evidence for supra-subduction zone formation and extensive melt-rock interaction

Scott A. Whattam, Moonsup Cho, Ian E.M. Smith

Research output: Contribution to journalArticlepeer-review

36 Citations (Scopus)

Abstract

The Andong Ultramafic Complex (AUC) mainly comprises peridotites (wehrlites±plagioclase or spinel; or plagioclase+spinel) and related serpentinites with subordinate low-Al pyroxenites (clinopyroxenites, orthopyroxenites, and websterites). These rocks are compositionally similar to sub-continental lithospheric mantle peridotites and pyroxenites. Wehrlites formed predominantly by fractional crystallization processes within supra-subduction zone magmas and the pyroxenites are generally consistent with segregation and accumulation in similar magmas. Bulk rock ratios of Al2O3/SiO2 (0.01-0.03) and MgO/SiO2 (up to >1) exhibited by the wehrlites and serpentinites indicate crystallization from a refractory source that underwent high degrees of melt extraction. Spinel chemistry confirms this and demonstrates that wehrlite and clinoproxenite protoliths underwent approximately 20-23% and 12-15% partial melting, respectively. Wehrlites and serpentinites also preserve evidence of extensive melt-peridotite interaction manifest as bulk rock SiO2-depletions and FeOt-enrichments relative to mantle residua as well as low Mg# (0.39-0.45) spinels with variable Ti contents but constant Cr# (0.42-0.47). These features are identical to those of 'impregnated' plagioclase-peridotites of abyssal and sub-continental environments and compositional trends in spinel space imply reaction between secondary, MORB-like melts saturated in olivine+clinopyroxene or olivine and a harzburgitic protolith. High olivine:pyroxene (~3:1) and clinopyroxene:orthopyroxene ratios of the wehrlites coupled with chemical data dictate that reactions entailed orthopyroxene dissolution and olivine recrystallization. All AUC rock types exhibit primitive mantle-normalized incompatible element signatures characterized by LILE-enrichments, high fluid-mobile/immobile element ratios (Sr/Nd, Ba/La and Pb/Ce>1) and prominent HFSE (Nb, Zr, and Ti) depletions indicative of generation in a sub-arc environment within a supra-subduction zone system. A candidate for the associated arc-system is the one responsible for nearby arc-related Jurassic granitoids. Southeast-directed thrusting along the Andong Fault System may account for subsequent emplacement of the AUC into the Gyeongsang Basin.

Original languageEnglish
Pages (from-to)599-618
Number of pages20
JournalLithos
Volume127
Issue number3-4
DOIs
Publication statusPublished - 2011 Dec

Bibliographical note

Funding Information:
Editor A. Kerr and reviewers S. Arai and O. Parlak are thanked for detailed and constructive comments which greatly aided in revision of the original version of the manuscript. M. Lee, SNU probe technician, is thanked for her assistance in the operation of and acquisition of data from the JEOL 8900 electron microprobe. Y. Lee is thanked for assistance in the field and J.K. Kim for assistance in data manipulation. S.A. Whattam thanks R.J. Stern for comments and ideas related to this study. This work was supported in part by a NRF-MEST grant 2010-0000296 to M. Cho and a postdoctoral fellowship to S.A. Whattam via the Brain Korea (BK) 21 grant, Seoul National University .

Keywords

  • Melt-rock reaction
  • Peridotite
  • South Korea
  • Supra-subduction zone (SSZ)
  • Ultramafic magmas
  • Volcanic arc

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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