Tracking decratonization process along a cratonic edge through late Permian to late Triassic magmatic flare-up in northwestern Liaoning, North China Craton
Tracking decratonization process along a cratonic edge through late Permian to late Triassic magmatic flare-up in northwestern Liaoning, North China Craton
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追踪辽宁西北部华北克拉通晚二叠世至晚三叠世岩浆爆发沿克拉通边缘的去克拉通化过程
DOI：
10.1016/j.lithos.2020.105916
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发表时间：
2021
期刊：
影响因子：
3.5
通讯作者：
Yuan Lingling
中科院分区：
文献类型：
--
作者：
Yang Zhili;Zhang Xiaohui;Yuan Lingling
在任何区域构造和超大陆重建中，沿会聚板块边缘的多尺度岩浆速度的确定都是一项关键任务，对于古生代至早中生代与中亚造山带会聚的华北克拉通（NCC）北部具有至关重要的意义。本文应用锆石U-Pb综合测年和地球化学示踪对辽西北晚二叠世—晚三叠世的侵入耀斑进行了研究，破译了3期花岗岩套。它们对比鲜明的元素和同位素特征表明，它们的形成涉及三种不同的地壳端元：古角闪原岩、古生代增生的基性变火成岩成分和复层状的幼年中间弧岩体。在长英质热区，这些多重并置原岩的序贯地壳深熔倾向于产生连续的母岩浆，形成三个花岗岩套。晚二叠世（260 ~ 250 Ma）花岗岩具有长英质阿达质亲和性，同位素组成变化(87Sr/86Sri= 0.70637 ~ 0.70659, εNd(t) = - 16.7 ~ - 10.2，锆石εHf(t) = - 18.1 ~ +1.1)，与主要古地壳物质部分熔融和少量新下镀基性成分相一致，而中三叠世（242 ~ 240 Ma）花岗岩的同位素特征更为年轻（εNd(t) = - 11.8 ~ - 4.7）。锆石εHf(t) =−14.7 ~ +4.1)，证明其形成过程中新下镀基性下地壳的输入明显增加。晚三叠世（227 ~ 223 Ma）花岗岩表现为a型岩浆亲和。其放射性成因全岩Nd（εNd(t) =−3.7 ~ +1.1）和锆石Hf（εHf(t) = +0.7 ~ +12.0）同位素值表明岩浆来源于以复层状为主的幼年炭质原岩的高温融合。随后是一阶段的闪玄岩基性岩脉，其特征为贫同位素组成（87Sr/86Sri= 0.70374 ~ 0.70388, εNd(t) = +3.5 ~ +4.1）的板熔交代软流圈源。将这些事件与前人发现的事件综合起来，得到了约260 ~ 218 Ma的一级爆发持续时间，并进一步划分为约260 ~ 250 Ma的斜长岩和花岗岩、约242 ~ 238 Ma的铁质花岗岩和约227 ~ 218 Ma的a型花岗岩和基性岩脉三个次级脉冲。虽然这种初级持续时间是大陆边缘会聚后碰撞/造山期岩浆速度的典型特征，但三个次级脉冲包含了古亚洲大洋闭合后从地壳增厚到造山期崩塌到岩石圈滴流的连续地球动力学过程。辽西北地区的克拉通边缘破坏具有岩石圈减薄和地壳生长的双重特征，是晚中生代华北盆地东部可能由于古太平洋板块的大洋俯冲而发生的大规模去克拉通化的前导和对照。
Resolving magmatic tempos of multiple scales along convergent plate margins presents a pivotal task in any regional tectonic and supercontinental reconstructions, as is of paramount importance for the northern North China Craton (NCC) that converged with the Central Asian Orogenic belt (CAOB) during the Paleozoic to early Mesozoic. This study applies the integrated zircon U–Pb dating and geochemical tracing to late Permian to late Triassic intrusive flare-up in northwestern Liaoning, with three episodes of granitic suites deciphered. Their contrasting elemental and isotopic features called for three distinct crustal end-members involved in their formation: ancient amphibolitic protolith, Paleozoic-accreted mafic meta-igneous ingredients and relaminated juvenile intermediate arc plutons. Sequential crustal anatexis of these multiply juxtaposed protoliths in a felsic hot zone tends to yield successive parental magmas for forming three granite suites. While the late Permian (260–250 Ma) granites, with their felsic adakitic affinity and variably evolved isotopic compositions (87Sr/86Sri= 0.70637 to 0.70659, εNd(t) = −16.7 to −10.2, zircon εHf(t) = −18.1 to +1.1), are consistent with partial melts of major ancient crustal materials and minor newly-underplated mafic ingredients, the middle Triassic (242–240 Ma) granites show a more juvenile isotope signature (εNd(t) = −11.8 to −4.7, zircon εHf(t) = −14.7 to +4.1) and attest to distinctively higher input from newly-underplated mafic lower crust in their formation. By contrast, the late Triassic (227–223 Ma) granites exhibit an A-type magmatic affinity. Their radiogenic whole-rock Nd (εNd(t) = −3.7 to +1.1) and zircon Hf (εHf(t) = +0.7 to +12.0) isotopic values suggest magma derivation from high-temperature fusion of dominantly relaminated juvenile charnockitic protolith. What follows is one phase of shoshonitic mafic dykes that fingerprint a slab-melt metasomatized asthenosphere source with depleted isotopic compositions (87Sr/86Sri= 0.70374 to 0.70388, εNd(t) = +3.5 to +4.1). Synthesizing these events with the previously recognized ones leads to a first-order duration of ca. 260–218 Ma for the flare-up and its further differentiation into three secondary pulses including ca. 260–250 Ma appinite and granite, ca. 242–238 Ma ferroan granites and ca. 227–218 Ma A-type granites and mafic dykes. While such primary duration is typical of post- collisional/orogenic magmatic tempos of convergent continental margins, three secondary pulses encapsulate successive geodynamic processes from crustal thickening through orogenic collapse to lithospheric dripping in the aftermath of the Paleo-Asian oceanic closure. Featuring a coupled scenario of lithospheric thinning and crustal growth, the northwestern Liaoning case of cratonic-edge destruction serves as a prelude but also a contrast to the late Mesozoic large-scale decratonization across the eastern NCC possibly due to oceanic subduction of the paleo-Pacific plate.
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