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新发现榴辉岩带位于拉萨北东方向,产在青藏高原拉萨地体中东部。观察到的榴辉岩带宽度约500~600m,呈近东西向延伸,已知规模10km以上。榴辉岩为常见的金红石榴辉岩、石英榴辉岩和多硅白云母榴辉岩。榴辉岩的单斜辉石中含硬玉分子变化较大,Jd=16%~44%,但均落于绿辉石成分区间;石榴子石中镁铝榴石(Pyrope)端元组分16%~33%,铁铝榴石(Alm)40%~54%,钙铝榴石(Gross)22%~31%。利用Grt-Omp-Phe和Grt-Omp矿物组合对变质温-压初步估算,获得金红石榴辉岩样品06Y-334的变质p、t分别为2.58GPa、635℃和2.67GPa、730℃,样品06Y-345的t主要在680~780℃区间,样品06Y-336的t主要在640~740℃区间,3个样品获得的结果相近。显微镜观察表明多硅白云母可能为折返阶段的退变质矿物而不属变质峰期的矿物,推测峰期的压力值有可能高于2.67GPa而进入柯石英稳定区间(p>2.8GPa)。石榴子石和绿辉石中出现的一些类似柯石英假象的石英包裹体与这一推断相吻合。以上初步研究结果表明,拉萨地体的榴辉岩带可能是中国境内又一条高压/超高压(?)变质带。
The newly discovered eclogite belt is located in the northeastern Lhasa direction, produced in the eastern part of Lhasa terrane of the Qinghai-Tibet Plateau. The observed eclogite belt width is about 500-600 m, extending nearly east-west, with a known scale of over 10 km. Eclogites are common eclogite eclogites, quartz eclogite and plutonic eclogites. The eclogitic pyroxenes in the eclogite have large changes in the content of jadeite, with Jd = 16% ~ 44%, but they all fall within the omphacite interval. The pyrope endmember component 16 % To 33%, Almandine 40% to 54%, and Almandine 22% to 31%. Using the combination of Grt-Omp-Phe and Grt-Omp mineral assemblages, the metamorphic temperature-pressure was used to estimate the metamorphic p-value of 06Y-334 in the Au-garnet eclogites. The pt values were 2.58GPa, 635 ℃ and 2.67GPa, respectively. The t of 06Y-345 was mainly in the range of 680-780 ℃. The t of sample 06Y-336 was mainly in the range of 640-740 ℃. The results of three samples were similar. Microscopic observation showed that the plutonic mica may be the retrograde metamorphic mineral without metamorphic peak, suggesting that the peak pressure may be higher than 2.67GPa and enter the stable region of coesite (p> 2.8GPa). Some of the quartz inclusions in the garnet and omphacite similar to the coesite artifacts coincide with this assumption. The above preliminary results show that the eclogite belt of Lhasa terrane may be another high-pressure / ultra-high pressure (?) Metamorphic belt in China.