Counter-clockwise prograde P–T path in collisional orogeny and water subduction at the Precambrian–Cambrian boundary: The ultrahigh-pressure pelitic schist in the Kokchetav massif, northern Kazakhstan
Counter-clockwise prograde P–T path in collisional orogeny and water subduction at the Precambrian–Cambrian boundary: The ultrahigh-pressure pelitic schist in the Kokchetav massif, northern Kazakhstan
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DOI：
10.1016/j.gr.2008.07.001
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发表时间：
2009-04
影响因子：
6.1
通讯作者：
H. Masago;S. Omori;S. Maruyama
中科院分区：
文献类型：
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作者：
H. Masago;S. Omori;S. Maruyama
利用粗粒和富含包裹体的石榴石中的成分分带和矿物包裹体，估算了哈萨克斯坦Kokchetav超高压区最新前寒武纪泥质片岩的P-T路径。石榴子石中含钛包裹体丰富，呈带状分布。钛铁矿产于内核，其中大部分与金红石形成复合包裹体，而单矿物金红石则产于外核至地幔域。在边缘区域，钛铁矿和金红石都存在，尽管数量很少。钛铁矿-石榴石温度计的应用产生了从500 ° C到750 °C的朝向边缘的系统温度升高。压力敏感反应：3Fe-Ilm（以Ilm为单位）+Ky+2 Qtz= 3Rt +Alm（以Grt为单位）得到外核包裹体的压力为1.2- 1.3GPa。利用K_2 O-CaO-FeO-MgO-Al_2O_3-SiO_2-H_2O模型系统中的成岩网格估算了石榴子石的平衡成分。正演模拟中预期的钙铝榴石组分沿着模型P-T路径的变化与观察到的外核-边缘区域的成分剖面接近。没有约束是可从热压法在内核，然而，石榴石分区的正演模拟提供的信息在石榴石生长的早期阶段的P-T路径。在700 °C和1.2-1.5 GPa附近，P-T路径为逆时针方向，并有一个陡峭的弯曲。这类似于Kokchetav变质带的P-T梯度。这一结果与许多碰撞变质岩石的顺时针P-T轨迹形成鲜明对比，并被认为代表了前寒武纪-寒武纪界线的俯冲地热。研究中提出的P-T路径也支持了通过地幔水的作用从晚元古代冰川作用中恢复“雪球地球”的模型。
A prograde pressure–temperature (P–T) path is estimated for pelitic schists from the latest Precambrian Kokchetav ultrahigh-pressure massif, Kazakhstan, using compositional zoning and mineral inclusions in coarse-grained and inclusion-rich garnets. Ti-bearing inclusions are abundant in garnet and display a zonal distribution. Ilmenite occurs in the inner-core, where most of it makes a composite inclusion with rutile, whereas monomineralic rutile occurs in the outer-core to mantle domains. In the rim region both ilmenite and rutile are present, although in small amounts. Application of the ilmenite-garnet thermometer yields a systematic temperature increase towards rim from 500 to 750 °C. The pressure-sensitive reaction: 3 Fe-Ilm (in Ilm)+Ky+2 Qtz=3 Rt+Alm (in Grt) yielded pressures of 1.2–1.3 GPa for the outer-core inclusions. A petrogenetic grid in the K2O–CaO–FeO–MgO–Al2O3–SiO2–H2O model system was used to estimate the equilibrium compositions of the garnet. The change of the grossular component along the model P–T path expected from the forward modelling is close to the observed compositional profile of the outer-core to rim domains. No constraint is available from thermobarometry in the inner-core; however, the forward modelling of garnet zoning provides information on the early stage of the P–T path during the garnet growth. The estimated P–T path is counter-clockwise in the prograde stage with a steep bend at around 700 °C and 1.2–1.5 GPa. This is similar to the metamorphic P–T gradient of the Kokchetav massif. This result contrasts markedly with the traditional clockwise P–T path in many collisional metamorphic terranes, and is regarded to represent a subduction geotherm at the Precambrian–Cambrian boundary. The P–T path proposed in this study also supports the models for the recovery of the “snowball Earth” from late-Proterozoic glaciation through effect of water in the solid Earth mantle.