Pseudosection modelling and garnet Lu-Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet
Pseudosection modelling and garnet Lu-Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet
复制标题
高压角闪石片岩的拟剖面模型和石榴石Lu-Hf年代学限制了西藏中部拉萨地块北部和南部之间的洋盆闭合
DOI：
10.1111/jmg.12255
复制
发表时间：
2017
影响因子：
3.4
通讯作者：
Zhang L.
中科院分区：
文献类型：
--
作者：
Cao D. D.;Cheng H.;Zhang L.
俯冲带中高变质岩石的P-T-t轨迹可能揭示沿着会聚板块边界的构造过程的有价值的信息。本文介绍了西藏拉萨地块几种亲洋角闪石片岩的岩石学、假剖面模拟和放射性定年研究。角闪石片岩经历了一个整体的顺时针P-T路径，其特征是在达到峰值高温和低温条件（约490°C和1.6 GPa）后，经历了后Pmax加热-减压和随后的等温减压。假剖面模拟表明，角闪石片岩在退变质作用中经历了水不饱和状态，在水不饱和状态下，稳定性场沿沿着退变质路径向相对于水饱和状态的低温高压方向扩展，高钾角闪石片岩在退变质作用中高度还原。从石榴石和基质绿帘石中绿帘石包裹体的不同成分确定的三价铁条件的精确评价是通过石榴石等温热压法进行真实P-估计的关键。对含石榴子石角闪石片岩中石榴子石粒度分离物的Lu-Hf同位素分析得到了四个两点石榴子石全岩等时线年龄，从228.2 ± 1.2 Ma到224.3 ± 1.2 Ma。这些Lu-Hf年龄被解释为限制了石榴石生长的时期，并近似于榴辉岩变质作用的时间，因为岩石的峰值变质温度较低，石榴石中的Mn/Lu生长带保存完好。大多数锆石U-Pb年龄对变质作用的时间没有限制，但是，从狭窄和不常见的变质边缘获得了两个一致的U-Pb年龄，分别为222.4 ± 3.9 Ma和223.3 ± 4.2 Ma。锆石和辉石的共存表明变质锆石的生长可能是在折返过程中由金红石向辉石的退变质作用的结果。锆石U-Pb和石榴子石Lu-Hf放射性年龄几乎一致，表明角闪片岩的快速埋藏和折返，表明闭合时间为c。拉萨地块北方和南方之间的化石洋盆为224-223 Ma。
TheP–T–tpath of high‐Pmetamorphic rocks in subduction zones may reveal valuable information regarding the tectonic processes along convergent plate boundaries. Herein, we present a detailed petrological, pseudosection modelling and radiometric dating study of several amphibole schists of oceanic affinity from the Lhasa Block, Tibet. The amphibole schists experienced an overall clockwiseP–Tpath that was marked by post‐Pmaxheating–decompression and subsequent isothermal decompression following the attainment of peak high‐Pand low‐Tconditions (~490°C and 1.6 GPa). Pseudosection modelling shows that the amphibole schists underwent water‐unsaturated conditions during prograde metamorphism, and the stability field of the assemblage extends to lower temperatures and higher pressures within the water‐unsaturated condition relative to water‐saturated model along the prograde path. The high‐Pamphibole schists were highly reduced during retrograde metamorphism. Precise evaluation of the ferric iron conditions determined from the different compositions of epidote inclusions in garnet and matrix epidote is crucial for a trueP–Testimate by garnet isopleth thermobarometry. Lu–Hf isotope analyses on garnet size separates from a garnet‐bearing amphibole schist yield four two‐point garnet–whole‐rock isochron ages from 228.2 ± 1.2 Ma to 224.3 ± 1.2 Ma. These Lu–Hf dates are interpreted to constrain the period of garnet growth and approximate the timing of prograde metamorphism because of the low peak metamorphic temperature of the rock and the well‐preserved Mn/Lu growth zoning in garnet. The majority of zircon U–Pb dates provide no constraints on the timing of metamorphism; however, two concordant U–Pb dates of 222.4 ± 3.9 Ma and 223.3 ± 4.2 Ma were obtained from narrow and uncommon metamorphic rims. Coexistence of zircon and sphene in the samples implies that the metamorphic zircon growth was likely assisted by retrogression of rutile to sphene during exhumation. The near coincident radiometric dates of zircon U–Pb and garnet Lu–Hf indicate rapid burial and exhumation of the amphibole schists, suggesting a closure time ofc. 224–223 Ma for the fossil ocean basin between the northern and southern Lhasa blocks.