Accretionary wedge harzburgite serpentinization and rodingitization constrained by perovskite U/Pb SIMS age, trace elements and Sm/Nd isotopes: Case study from the Western Carpathians, Slovakia

• Published in: Lithos 2014-09, Vol.205, p.1-14
• Main Authors: Li, Xian-Hua, Putiš, Marián, Yang, Yue-Heng, Koppa, Matúš, Dyda, Marian
• Format: Article
• Language: English
• Subjects: Accretionary wedge; Perovskite U/Pb SIMS age; Rodingitization; Serpentinization; Western Carpathians
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2014.06.001

Perovskite-bearing harzburgites occur in a “mélange” type blueschist-bearing accretionary wedge complex of the Inner Western Carpathians Meliata Unit in Slovakia. Although dark rounded, slightly hydrated relic “cores” of harzburgite boulders are perovskite-free, perovskite (Prv) occurrence in the surrounding serpentinites and rodingites enabled dating of hydration, resulting in two metamorphic–metasomatic Prv generations. Perovskite (1) grows parallel to relic clinopyroxene exsolution lamellae or forms randomly oriented grain clusters in serpentinized orthopyroxene (Opx1) porphyroclasts, often accompanied by tiny andradite lamellae clusters, or it is partly replaced by Ti-andradite. Perovskite crystallization indicates evolving rodingitization fluids pervading the boundary between the harzburgite “cores” and Prv-free serpentinite. This strictly limited occurrence of Prv (1) within a 1 to 20-cm across-zone implies slightly postponed Prv crystallization to serpentinization by LREE(Ce,La), Ca2+, Ti/Fe3+-enriched aqueous fluids. A grain scale metasomatic mechanism partitioned Ca and Ti from the host orthopyroxene porphyroclasts, spinel (Ti) and grain-boundary pervasive fluids to Prv. In contrast, Prv (2) occurs in a 1 to 3cm across chlorite-rich blackwall zone between hosting serpentinite and rodingite veins, thus indicating channelled rodingitization fluid flow and accompanying hydraulic fracturing. Here, Prv (2) is ingrown by chlorite and apatite. Part of this Prv (2) formed in a rodingite vein mineral assemblage composed of diopside, andradite, vesuvianite, epidote/zoisite, apatite and chlorite. Both perovskite 1 and 2 are replaced by pyrophanite along the grain rims and interiors; most likely via fluid-aided coupled dissolution–reprecipitation at increased Si–Fe–Mn–Al element solubility in rodingitization fluids pervading serpentinized harzburgite. Both Prv generations, especially Prv (2), can be partly to almost totally replaced by (Ti-) Adr. Overgrowths of spinel by andradite are occasionally observed in contact zones between the serpentinites and rodingites. LA-ICP-MS study revealed strong depletion in LREE from Prv (1) to Prv (2), and a more typically positive Eu anomaly for Prv (2). Our spider diagram depicts relative enrichment in U, Nb, La, Ce, Pr, Nd, and decreased Rb, Ba, Th, Ta, Pb, Sr, Zr in both Prv generations. The U/Pb SIMS concordia ages of Prv (1) from 3 samples range from 137±1Ma to 135±1Ma, with a mean of 135.6±0.58Ma, while Prv (2) was d