摘要
本文以大别山双河柯石英榴辉岩为基础,在NCKMnFMASHO体系中计算了P-T、T-M(H2O)和P-M(H2O)一系列相图。这些相图表明在地壳冷俯冲(地热梯度约为6℃·km-1)过程中饱和水的变质基性岩通过脱水反应导致矿物组合演化,随着P-T条件增加,约在2.2GPa(80km)处,绿泥石被滑石取代,在2.4GPa处,蓝闪石消失,在2.5GPa处,滑石消失,在2.9GPa(105km)处,硬柱石消失。相应地可出现硬柱石蓝片岩、蓝闪石-硬柱石榴辉岩、硬柱石榴辉岩和多硅白云母榴辉岩等高压-超高压组合。俯冲基性岩中即使含有很少量的水(如0.3%～0.5%),都会在超高压硬柱石榴辉岩相条件下达到饱和。因此硬柱石会广泛出现于经历冷俯冲地壳的变质基性岩中。高压-超高压榴辉岩的折返过程受折返温度及峰期矿物组合的控制。当峰期矿物组合中含有硬柱石(±蓝闪石±滑石,T=540～600℃)时,其早期折返发生在硬柱石稳定域,受脱水反应控制,难以保存峰期矿物组合;晚期折返发生在绿帘石稳定域,岩石处于流体缺失状态,有利于保存硬柱石消失后的高压矿物组合。当峰期矿物组合中含有绿泥石、硬柱石和蓝闪石时(T<540℃),其折返过程中的脱水作用仅发生在硬柱石与绿帘石共生的狭窄区域,在硬柱石稳定域的早期折返与绿帘石稳定域的晚期折返阶段,都不发生脱水作用,岩石处于流体缺失状态,因此,虽然峰期形成的硬柱石难以保存,但峰期形成的其他矿物可能仅受轻微改造。当硬柱石消失(T>600℃)后,多硅白云母高压-超高压榴辉岩中含有很少量水,在早期折返过程中的很大压力范围内,岩石保持水含量不变,更容易保留峰期矿物组合。高压-超高压榴辉岩在减压至1.5GPa以下时,由于外来流体注入,发生部分水化,形成含有钠钙质、钙质角闪石榴辉岩,它们一般不是平衡矿物组合。榴辉岩中名义上的无水矿物在减压过程中释放的水有助于榴
Based on coesite-bearing eclogites from Shuanghe of Dabieshan terrane,pseudosections including P-T、T-M （H2O） and P-M （H2O） were calculated in the NCKMnFMASHO system.Phase equilibria modeling predicts that the mineral assemblages in water-saturated metabasic rocks evolve via dehydration in cold crustal subduction zones （geothermal gradient of about 6℃·km-1）.With increasing P-T conditions,chlorite is replaced by talc at about 2.2GPa （80km）,and then glaucophane,talc and lawsonite disappear at 2.4GPa,2.5GPa and 2.9GPa （105km）,respectively.Accordingly,high-ultrahigh pressure （HP-UHP） mineral assemblages such as lawsonite-bearing blueschist,glaucophane-lawsonite eclogite,lawsonite eclogite and phengite eclogite are modeled to be presented.Phase equilibria modeling also suggests that even the subducted basic rocks contain quite small quantities of water （such as 0.3%~0.5%）,they can also be water-saturated under UHP lawsonite eclogite facies.Thus,lawsonite will be widely present in the metabasic rocks which have experienced the cold subduction.The exhumation of HP-UHP eclogites is controlled by the temperature and the peak mineral assemblage.When the peak eclogitic assemblage contains lawsonite （±glaucophane±talc） at T=540~600℃）,it will involve two stages of exhumation：The early-stage exhumation is modeled to proceed in the lawsonite stability field,controlled by dehydration reactions,and thus,it will be difficult to preserve the peak mineral assemblage.The late-stage exhumation proceeds in the epidote stability field where the rock is fluid-absent and the HP eclogitic assemblage after the lawsonite disappearance is prone to be preserved.When the peak mineral assemblage includes chlorite,lawsonite and glaucophane at T540℃,the decompression-driven dehydration reactions only occur in the narrow area with the coexistence of lawsonite and epidote,and will not be happened either in the early exhumation with lawsonite stability and the late exhumation with epidote stability,where
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2011年第4期1067-1074,共8页
Acta Petrologica Sinica
基金
国家自然科学基金资助项目(40972039
40821002)
科技部973项目(2009CB825007)联合资助
