Very Low- to Low-grade Metamorphic Processes Related to the Collisional Assembly of Avalonia in SE Cape Breton Island (Nova Scotia, Canada)
Very Low- to Low-grade Metamorphic Processes Related to the Collisional Assembly of Avalonia in SE Cape Breton Island (Nova Scotia, Canada)
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与布雷顿角岛东南部阿瓦隆尼亚碰撞组装相关的极低到低级变质过程（加拿大新斯科舍省）
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发表时间：
2013
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影响因子：
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通讯作者：
C. White
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作者：
A. Willner;H. Massonne;S. Barr;C. White
为了更好地理解与东南部布雷东岛阿瓦隆米拉火山岩（原岩年龄680 ~ 560 Ma）中不同火山岩带、沉积岩带组合有关的地球动力学过程，我们研究了20个含白云母的镁铁质和长英质变火山岩的变质过程。长英质变火山岩具片理化特征，部分受变斑残余组构剪切，含多硅白云母、绿帘石、钠长石、钾长石、石英、钛硬锰矿、方解石、钛铁矿等组合。相比之下，许多镁铁质变火山岩相对未变形，并显示残留斑状和杏仁状结构。它们包括绿帘石、钠长石、石英、钛多硅白云母、绿帘石、葡萄石、方解石、钾长石和阳起石、绿帘石、钠长石、石英、钛多硅白云母、方解石、钾长石组合。局部残留的岩浆单斜辉石、磁铁矿和斜长石等指示了不均匀变质叠加作用。变质矿物是由局部沉淀成簇形成的，是由于在250 ^3008 ℃的强烈脱水过程中释放出的渗透性流体的脉冲过程中，非常低级的相连续成核而形成的。在这些条件下，成核速率占主导地位的增长速度。镁铁质和长英质岩石中的钾质白色云母大多为多硅白云母，其成分范围较宽（3·11 ^3·41 Si a.p.f.u.）。最大Si含量通常在3·30和3·41a.p.f.u.之间。在200^4008C和1^7千巴的范围内计算了P^T假截面。峰期变质组合所处的P^T场与白色云母中硅含量最大值的等值线位置一致。Mira P_1 ~ T峰值条件在3.5kbar ~ 0.4kbar范围内，代表了所有火山-沉积带组合的样品的温度为280 ~ 308 ℃。由此得出的峰期变质条件表明，同碰撞埋藏深度为11^14 km，低变质温度为20^258 C km。在这些条件下，随后的走滑变形归因于岩浆弧片的最后组装，形成了Mira-Mere的地壳。
To better understand geodynamic processes related to the assembly of various belts of volcanic^sedimentary rocks in the Avalonian Mira terrane of SE Cape Breton Island (protolith ages 680^560 Ma) we investigated metamorphic processes in 20 white-mica-bearing mafic and felsic metavolcanic rocks. The felsic metavolcanic rocks are foliated and partly sheared with blastoporphyritic relict fabric and contain the assemblage phengite^epidote^chlorite^ albite^K-feldspar^quartz^titanite stilpnomelane calcite ilmenite. In contrast, many mafic metavolcanic rocks are relatively undeformed and display relict porphyritic and amygdaloidal textures. They contain the assemblages epidote^chlorite^albite^quartz^titanite phengite pumpellyite prehnite calcite K-feldspar and actinolite^epidote^chlorite^albite^quartz^titanite phengite calcite K-feldspar. Heterogeneous metamorphic overprinting is indicated by local relicts of magmatic clinopyroxene, magnetite and plagioclase. Metamorphic minerals formed by local precipitation in clusters and are due to continuous nucleation of very low-grade phases during pulses of variably pervasive fluids, which were released during intense dehydration at 250^3008C. Nucleaction rate dominated over growth rate at these conditions. Potassic white mica in both mafic and felsic rocks is mostly phengite with a wide compositional range (3·11^3·41 Si a.p.f.u.). Maximum Si contents are typically between 3·30 and 3·41a.p.f.u. P^T pseudosections were calculated for the range 200^4008C and 1^7 kbar. The peak metamorphic assemblages occupy P^T fields consistent with the position of isopleths for corresponding maximum Si contents in white mica. Peak P^Tconditions in the Mira terrane samples lie within a narrow range of 3·5 0·4 kbar and 280 308C in samples representing all of the assembled volcanic^sedimentary belts. The derived peak metamorphic conditions suggest syn-collisional burial to 11^14 km depth and a low metamorphic geotherm of 20^258C km. Under these conditions subsequent strike-slip deformation is attributed to the final assembly of magmatic arc slices to form the crust of the Mira terrane.