纽芬兰阿巴拉契亚山脉东部和南部与阿卡迪亚造山周期相关的志留纪至泥盆纪岩浆岩演化:以花岗岩岩浆的偶发性腹地和前陆定向迁移为特征的三阶段演化证据

Chaoyang Wang, Tao Wang, C. V. van Staal, Zengqian Hou, Shoufa Lin
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引用次数: 0

摘要

岩浆活动随时间的迁移和特征可为了解造山运动的构造演化提供重要信息。我们提出了与纽芬兰阿巴拉契亚山脉东部和南部阿卡迪亚造山周期相关的三个不同阶段的花岗岩岩浆活动的证据。这些解释基于来自 15 个志留纪和泥盆纪花岗岩岩体的 18 个样本的新锆石 U-Pb 年龄、地球化学数据和 Sr-Nd-Hf-O 同位素数据,并结合了以前发表的数据。这三个阶段概述了腹地和前陆导向的迁移趋势,代表了阿卡迪亚造山运动周期中的俯冲(435-420 Ma)、同步碰撞(415-405 Ma)和后碰撞(395-370 Ma)环境。第一阶段的志留纪深成岩(435-420 Ma)主要由石英闪长岩、辉绿岩、花岗闪长岩、单斜花岗岩和正长花岗岩组成,具有高K钙碱性和富集的Sr-Nd-Hf-O同位素组成(例如,εNd[t] = -5 to -2;εHf[t] = -3 to -1; δ18O = +6‰ to +8‰)。据解释,它们记录了阿卡迪亚海道海洋岩石圈的俯冲,该海道将以甘德边缘为代表的复合劳伦西亚前缘与阿瓦隆尼亚分隔开来。第二阶段早泥盆世的长岩(415-405 Ma)含有更多的单斜花岗岩和正长花岗岩;它们具有钙碱性至高钾钙碱性特征、类似金刚石的成分以及更贫化的Sr-Nd-Hf-O同位素成分(例如,εNd[t] = -6 to 0;εHf[t] = +1 to +3;δ18O = +5‰ to +6‰)。这一阶段的岩浆岩主要出现在志留纪花岗岩的西北部,表明岩浆活动以大于9 km/m.y的速度在区域范围内向西北方向(腹地方向)迁移。第三阶段(395-370Ma)的中泥盆世至晚泥盆世岩浆岩主要由单斜花岗岩、正长岩和碱长花岗岩组成,它们是富含硅和碱的花岗岩,具有较大的负Eu异常。这些岩石主要集中在多佛-赫米蒂奇湾断层带两侧,该断层带是阿瓦隆尼亚和复合劳伦西亚的分界线,位于志留纪和早泥盆纪火成岩的东南部。这一阶段的岩浆活动代表了前陆定向(后退)迁移。早泥盆世和中泥盆世至晚泥盆世的岩浆活动被 405 Ma 至 395 Ma 之间的间隙隔开,并记录了从(高钾)钙碱性向碱性成分的演变,这归因于碰撞后环境中阿瓦隆岩石圈地幔的部分脱层。
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Evolution of Silurian to Devonian magmatism associated with the Acadian orogenic cycle in eastern and southern Newfoundland Appalachians: Evidence for a three-stage evolution characterized by episodic hinterland- and foreland-directed migration of granitoid magmatism
The migration and character of magmatism over time can provide important insights into the tectonic evolution of an orogen. We present evidence for three separate stages of compositionally distinct granitoid magmatism associated with the Acadian orogenic cycle in the eastern and southern Newfoundland Appalachians. The interpretations are based on new zircon U-Pb ages, geochemical data, and Sr-Nd-Hf-O isotopic data for 18 samples from 15 Silurian and Devonian granitoid plutons, combined with previously published data. The three stages outline hinterland- and foreland-directed migration trends and represent subduction (435−420 Ma), syncollision (415−405 Ma), and postcollision (395−370 Ma) settings in the Acadian orogenic cycle. The Silurian plutons (435−420 Ma) of the first stage consist mainly of quartz diorite, tonalite, granodiorite, monzogranite, and syenogranite, with high-K calc-alkaline and enriched Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −5 to −2; εHf[t] = −3 to −1; δ18O = +6‰ to +8‰). They are interpreted to record the subduction of oceanic lithosphere of the Acadian seaway that separated the leading edge of composite Laurentia, represented by the Gander margin, and Avalonia. Early Devonian plutons (415−405 Ma) of the second stage contain more voluminous monzogranite and syenogranite; they have calc-alkaline to high-K calc-alkaline features, adakite-like compositions, and more depleted Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −6 to 0; εHf[t] = +1 to +3; δ18O = +5‰ to +6‰). Plutons of this stage occur mostly to the northwest of the Silurian granitoids, indicating a regional-scale northwestward (hinterland-directed) migration of magmatism with a rate of >9 km/m.y. The migration is interpreted to have been related to the progressive shallow underthrusting of Avalonia beneath the Gander margin (composite Laurentia) at least as far as 90 km inboard. The Middle to Late Devonian plutons of the third stage (395−370 Ma) consist mainly of monzogranite, syenogranite, and alkali-feldspar granite, which are silica- and alkali-rich granites with large negative Eu anomalies. These rocks are concentrated along both sides of the Dover−Hermitage Bay fault zone, which represents the boundary between Avalonia and composite Laurentia, to the southeast of the Silurian and Early Devonian igneous rocks. This stage of magmatism represents a foreland-directed (retreating) migration. The Early Devonian and Middle to Late Devonian episodes of magmatism were separated by a gap between 405 Ma and 395 Ma and recorded an evolution from (high-K) calc-alkaline to alkaline compositions, ascribed to partial delamination of Avalonian lithospheric mantle in a postcollisional setting.
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