|
YanTao Hao, University of Ferrara (Italy)
Massimo Coltorti, University of Ferrara (Italy)
Costanza Bonadiman, University of Ferrara (Italy)
Luigi Dallai, CNR (Italy)
Barbara Faccini, University of Ferrara (Italy)
Qunke Xia, University of Science and Technology of China (China)
|
|
The Subei basin is located east of the Tanlu fault, a major discontinuity which separates the Archean North China Craton from the Proterozoic Yangtze Craton. This area experienced widespread basaltic volcanism during Cenozoic. Minor amount of tholeiites were erupted in the early Paleogene, while more extensive xenolith-bearing alkali basalt was outpoured in the Neogene. Four localities, Panshishan (PSS), Lianshan (LS), Pingshan (PS) and Fangshan (FS), about 30 km The Subei basin is located east of the Tanlu fault, a major discontinuity which separates the Archean North China Craton from the Proterozoic Yangtze Craton. This area experienced widespread basaltic volcanism during Cenozoic. Minor amount of tholeiites were erupted in the early Paleogene, while more extensive xenolith-bearing alkali basalt was outpoured in the Neogene. Four localities, Panshishan (PSS), Lianshan (LS), Pingshan (PS) and Fangshan (FS), about 30 km apart, were sampled. Most of the xenoliths are lherzolites, although few websterites, harzburgites and pyroxenite are also found. Neither hydrous nor metasomatic secondary phases were observed. Textures vary between coarse-grained protogranular (∼70%), porphyroclastic (∼20%) and equigranular (∼10%) type. Rarely metasomatic textures, mainly spongy clinopyroxene, were observed. Modal cpx abundance in lherzolite vary from 16-17 vol% in the most fertile to ca. 5vol% in the most depleted. Thermobarometric estimates for PSS and LS show quite low equilibrium temperature (T=730-840°C), while PS have equilibrium temperature close to 960°C, and FS record a T between 1001°C and 1165 (but one value at 781°C). Mineral major element variation indicate a progressive depletion trend for the four localities. HREE contents in cpx from PSS and LS would indicate a depletion event in the spinel-facies, although few samples with Ybn<4xch could support an episode of melting in the garnet facies. Two different groups of cpx can be recognized in both localities, one characterized by a LREE-depleted ((La/Yb)n=0.35-0.71) and the other by a LREE-enriched ((La/Yb)n=10-20) patterns.
Typically the great majority of enriched cpx belongs to the most depleted samples. Quite pronounced Zr negative anomalies are shown by the depleted cpx (Zr/Zr*=0.4-0.9; Zr*=(Sm+Nd)/2), while enriched cpx may present more pronounced negative anomaly (Zr/Zr*=0.2-0.4) or slightly positive (Zr/Zr*=1.32-1.39), indicating a different nature of the metasomatizing melts.
FTIR analyses in PSS and LS opx and cpx were also carried out to determine the water content in these two nominally anhydrous minerals (NAM). Water contents in PSS cpx is higher (64-183ppm) than in LS cpx (37-102 ppm), while water content in opx is comparable in the two localities (16-61 in PSS; 13-45ppm in LS). As a consequence cpx/opx water partitioning is higher in PSS (2.97-6.40; average 4.08), with respect to those in LS (1.97-3.37; average 2.53). These latter values appears comparable to those observed in cpx and opx from Nushan and Hannouba xenoliths (ca. 2.2). Water content in NAMS show a fairly good correlation with the depletion degree, while no correlation was found with increasing degree of enrichment. This fact would support a pre-metasomatic origin for the water, which, in turn, would indicate an originally different content in the mantle domains.
|
