Garnet-Rich Granulite Xenoliths from the Hannuoba Basalts, North China: Petrogenesis and Implications for the Mesozoic Crust-Mantle Interaction

摘要

Garnet-rich granulite xenoliths collected from the Hannuoba basalts, the North China craton (NCC), were studied to reveal the Mesozoic crust-mantle interaction. These xenoliths are characterized by low SiO2 (37.7 wt.%–46.0 wt.%) and high Al2O3 (10.8 wt.%–17.9 wt.%) contents. Their Mg# (60–75, Mg#=100×Mg/(Mg+Fe), atomic number) are relatively low for their low SiO2 contents. They have low rare-earth element (REE) contents and LREE-rich REE patterns, and show remarkable enrichments in Sr relative to the adjacent REE. Some of them exhibit convex REE patterns with a maximum at Nd and remarkably positive Eu anomalies. Taking into account their high garnet mode (generally >30%), these features suggest that they are high-pressure metamorphic products of low-pressure cumulates (e.g., gabbro) after it had been depressed into the garnet stability field. They have evolved Nd and Sr isotopic composi-tions (143Nd/144Nd=0.511 763–0.512 173, 87Sr/86Sr=0.705 34–0.706 99) and fall in the trend defined by the >110 Ma Mesozoic basalts and high-Mg# andesites from the NCC. Zircon U-Pb dating by LA-ICP-MS shows a wide age range from 83 to 2 581 Ma, most of which cluster in 83–134 Ma. CL images of some Mesozoic zircons from the granulites show typical features of igneous zircons, providing direct evidence for the Mesozoic underplating event in this area. Neither peridotite-derived basaltic underplating model nor residue model of ancient lower crust after lithospheric thinning alone can reasonablyexplain the above features of the garnet-rich granulite xenoliths. Combined with the previous research, we propose that most of the granulite xenoliths from the Hannuoba basalts are products of the Mesozoic magmatic underplating and mixing with the pre-existing lower crust (i.e., AFC process). However, the melts could be mostly derived from partial melting of basaltic layers that were previously subducted (a fossil oceanic slab) or underplated into the base of the lithospheric mantle, or from partial melting of Archean lithospheric mantle that was variably hybridised by melts derived from foundered lower crustal eclogite, although it cannot be excluded that some of the melts were derived from depleted mantle peridotite. In other words, parent melts of most granulite xenoliths could share the same petrogenesis as the >110 Ma Mesozoic basalts from the NCC.