Mock eddy current demonstration: cracks vs. notches: Hagemaier, D.A. and Register, J.A. Mater. Eval. Jan. 1990 48, (1), 50–54

Abstract

This paper presents petrology, mineralogy, zircon U–Pb ages, and whole-rock major, trace element and Sr–Nd–Hf isotopic compositions of four Permian (273–253 Ma) subduction-related mafic intrusions (including the Qinggoushan and Qianshan gabbros, and the Wangqing and Shuguang diorites) from the Yanbian region, NE China, with aims to understand the role of subducted sediment flux in generation of arc mafic cumulates. These intrusions have mineral assemblages crystallized in water-saturated parental magmas and show variable degrees of crystal accumulation as observed in mafic cumulates in subduction zones. Mass-balance consideration indicates that their parental magmas were calc-alkaline with arc-type trace element features (enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depletions in Nb–Ta). They also have Sr–Nd–Hf isotopic compositions, i.e., 87Sr/86Sr(i) = 0.7029–0.7047, εNd(t) = + 0.9 ~ + 6.8, εHf(t) = + 5.6 ~ + 14.6, similar to modern arc basalts. The parental magmas were likely derived from a mantle wedge variably metasomatized by sediment melt and fluid from the subducting paleo-Asian Oceanic slab. Combined trace elemental and isotopic modeling results suggest that the parental magma of Qinggoushan gabbro was formed through 5–20% melting of the mantle wedge with 1% and 1.5% additions of sediment fluid and sediment melt, respectively; 5–10% melting of the mantle wedge through inputs of 1% sediment fluid and 2% sediment melt produced the Qianshan gabbro; 10–20% melting of the mantle wedge with additions of 1% sediment fluid and 3% sediment melt formed the Wangqing diorite; whereas 5–20% melting of the mantle wedge through an input of 1.5% sediment melt produced the Shuguang diorite.The Hf–Nd isotopic array of the Yanbian Permian mafic intrusions reflected the existence of an Indian Ocean-type mantle, which was isotopically distinct from the Pacific-type mantle during early Paleozoic in the Central Asian Orogenic Belt. Such a transformation from the Pacific-type to the Indian Ocean-type mantle was primarily attributed to addition of the recycled components, which contained more radiogenic Hf than Nd isotopic compositions, from the subducting paleo-Asian Oceanic slab.