P–T –t Path of Unusual Garnet–Kyanite–Staurolite–Amphibole Schists, Ellesmere Island, Canada—Quartz Inclusion in Garnet Barometry and Monazite Petrochronology
P–T –t Path of Unusual Garnet–Kyanite–Staurolite–Amphibole Schists, Ellesmere Island, Canada—Quartz Inclusion in Garnet Barometry and Monazite Petrochronology
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P – T – 异常石榴石之路 – 蓝晶石 – 十字石 – 角闪石片岩，加拿大埃尔斯米尔岛 – 石榴石气压计和独居石岩石年代学中的石英内含物
DOI：
10.1093/petrology/egac068
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发表时间：
2022
影响因子：
3.9
通讯作者：
Thomas, Jay B
中科院分区：
文献类型：
--
作者：
Kośmińska, Karolina;Gilotti, Jane A;McClelland, William C;Coble, Matthew A;Thomas, Jay B
奥陶纪火山碎屑岩中的石榴石-蓝晶石-十字石组合与大型晚期闪石斑岩形成，位于加拿大最北端埃尔斯米尔岛皮尔雅地体的南面。片岩与碳酸盐岩一起组成彼得森湾组合(PBA)，显示出一系列平行的等值线，标志着变质程度在与皮亚接触处10公里以上的距离上增加；然而，一条陡峭、脆性的新生代走滑断层以未知量的位移扰乱了早期的增生关系。角闪石的晚期生长，可能是由于流体进入，是石榴石中不平衡条件的明显证据。为了恢复片岩的P-T历史，我们建立了附近逃脱流体事件的石榴石-云母片岩的等化学相平衡模型，并将结果与石榴石(Quig)压力测量中的石榴石(Quig)包裹体的结果进行了比较。石英包裹体局限于石榴石岩心，Quig结果结合黑云母钛和石榴石黑云母测温，圈定了480~600°C和0.7~0.9 Gpa的进动路径。这条路径与石榴石中的石榴石成分的X射线图谱推算出的石榴石中的生长环带相吻合。用有效体积成分和石榴石边缘与变辉石中的基质黑云母和白云母等值线相交的伪截面模拟得到的峰值条件为665℃，≤为0.85GPa.通过对独居石的结构特征、稀土元素和Y元素含量的测定以及U-Pb年龄的测定，确定了独居石的三代(I、II、III)。独居石I以包裹体形式存在于基质中和石榴石边缘中，从尿蓝石碎裂开始，在397 ± 2 Ma(2σ)的峰温条件下生长。独居石II在基质独居石I颗粒上形成了与主片理面平行的过度生长，其屈服年龄为385 ± 2 Ma。独居石III仅见于石榴石中，其年龄为374 ± 6 Ma，解释为流体在较低温度和压力下沿顺时针方向的P-T路径生长，并保持在蓝晶石稳定区内。这些结果表明，一次相对较短的(≈12：Myr)巴罗纪变质事件影响了巴巴的片岩。毗邻的皮亚地体缺乏明显的热源，但我们推测，被走滑切割的是大型泥盆纪深成岩体-类似于位于距离断层40公里的390 ± 10 Ma开普伍兹花岗岩。与PBA相关的弧形碎片级别较低；它们从未见过热，也没有直接参与Pearya的吸积作用。
Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peakP–Tconditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwiseP–Tpath that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion.