汉诺坝地区上地幔尖晶石—石榴石相转变带温压条件

根据汉诺坝尖晶石石榴石二辉辉石岩包体矿物成分新资料和前人的尖晶石石榴石二辉橄榄岩包体矿物成分资料，运用斜方辉石Ｃａ溶解度温度计和斜方辉石－石榴石Ａｌ分配压力计，计算了尖晶石—石榴石相转变带的温度和压力条件，首次获得了汉诺坝地区的新生代古地温曲线。尖晶石石榴石二辉辉石岩包体代表的温度范围为９３０～９７８℃，压力范围为１４２～１６５ＧＰａ：尖晶石石榴石二辉橄榄岩包体代表的温度范围为９９１～１１１０℃，压力范围为１５８～２１６ＧＰａ，与最新的实验结果基本吻合。地温曲线沿大洋地温曲线（曲线方程为ｔ＝２５２４＋８９７４７８ｐ－１８３０８ｐ２）上方近平行延伸。推测汉诺坝玄武岩的起源深度至少为７０ｋｍ。二辉辉石岩包体来自较冷的岩石圈，而二辉橄榄岩来自岩石圈与软流圈的过渡带，并且表明后者的地温梯度以对流热地温梯度为主。

TEMPERATURE AND PRESSURE STATES OF SPINEL GARNET TRANSITION ZONE BENEATH HANNUOBA AREA

Based on new composition data on minerals of spinel garnet websterite xenolith and previous composition data on minerals of spinel garnet lherzolite xenolith from Hannuoba, the temperature and pressure states of spinel garnet transition zone are calculated by the combination of Ca in orthopyroxene thermometer and othorpyroxene garnet Al partition barometer getting the paleo geotherm of Cenozoic first time in this region. The ranges of temperature and pressure represented by spinel garnet websterite are 930～978℃ and 1 42～1 65GPa respectively, and those represented by spinel garnet lherzolite are 991～1110℃ and 1 58～2 16GPa respectively. The whole range of depth represented by these xonoliths is 45～70km which is roughly corresponding to the result of newly conducted experiment on spinel garnet transition. The geotherm curve, which is formulated as t (℃)=25 2+897 48 p +183 08 p 2(GPa), is above and super parallel with the ocean geotherm curve. It can be reckoned that xenolith bearing basalt originated at the depth of at least 70km. It may be suggested that websterite xenoliths came from cooler lithosphere and lherzolite ones from the depth near to lithosphere asthenosphere boundary, and the latter is also indicating a smaller geotherm gradient that is nearly close to typical advective geotherm gradient.