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Re-initiation of plutonism at the Gondwana margin after a magmatic hiatus: The bimodal Permian-Triassic Longwood Suite, New Zealand
Gondwana Research ( IF 6.1 ) Pub Date : 2021-10-05 , DOI: 10.1016/j.gr.2021.09.021
A.J. McCoy-West 1, 2 , N. Mortimer 3 , K.W. Burton 4 , T.R. Ireland 5 , P.A. Cawood 2
Affiliation  

The Cambrian to Cretaceous Tuhua Intrusives, New Zealand, preserve an igneous record of Phanerozoic subduction and crustal growth at the margin of Gondwana. Within the Tuhua Intrusives, the coeval gabbroic and trondhjemitic intrusions of the c. 261-243 Ma Longwood Suite stand out as being isotopically more primitive and chemically distinct from all other New Zealand plutonic suites. We present new U-Pb crystallization ages, trace element analyses and Sr-Nd isotope compositions of the Longwood Suite. U-Pb SHRIMP zircon ages of 258.5 ± 2.5 Ma, 256.0 ± 1.8 Ma, 247.8 ± 2.7 Ma and 243.2 ± 2.4 Ma obtained from plutons on Ruapuke Island, and a dike at Bluff, affirm the restricted time range and expand the known areal extent of the Longwood Suite. Longwood Suite granitoids are I-type and sodic (K/Na < 0.4), with distinctive low Rb and Nb/Ta, flat rare earth element patterns (La/YbN < 10), unradiogenic 87Sr/86Sr(t) (0.7029 to 0.7032) and radiogenic ε143Nd(t) (+6.3 to +8.2), compared to the nearby, calc-alkaline, Late Triassic Darran Suite I-type plutons of the Tuhua Intrusives. Stable Nd isotope ratios of Longwood Suite samples are highly variable (δ146/144Nd = 233 ppm) compared to global plutonic rocks (δ146/144Nd = 44 ppm) and reflect the removal of phosphate minerals. Collectively, these geochemical characteristics are consistent with generation of the granitoids by shallow (garnet-absent) melting of an amphibolitic residue, from which we infer relatively thin lithosphere. The Longwood Suite has a maximum areal addition rate of 43 km2/Ma, substantially less than the subsequent plutonic suites when the magmatic arc was fully established. We suggest a petrotectonic model whereby Gondwana continental margin crust was tectonically underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to generate the isotopically primitive gabbro and trondhjemite plutons of the Longwood Suite.



中文翻译:

岩浆中断后冈瓦纳大陆边缘深成体作用的重新启动:新西兰二叠纪-三叠纪长木双峰组

新西兰的寒武纪至白垩纪土化侵入岩保存了冈瓦纳大陆边缘显生宙俯冲和地壳生长的火成岩记录。在 Tuhua Intrusives 内,c. 261-243 Ma Longwood 组曲因其同位素更原始且化学性质与所有其他新西兰深成组曲不同而脱颖而出。我们展示了 Longwood Suite 的新 U-Pb 结晶年龄、微量元素分析和 Sr-Nd 同位素组成。从 Ruapuke 岛的岩体和 Bluff 的堤防中获得的 U-Pb SHRIMP 锆石年龄分别为 258.5±2.5 Ma、256.0±1.8 Ma、247.8±2.7 Ma 和 243.2±2.4 Ma,确认了限制时间范围并扩大了已知区域范围长木套房。Longwood Suite 花岗岩是 I 型和钠盐 (K/Na < 0.4),具有独特的低 Rb 和 Nb/Ta,N < 10)、非放射性87 Sr/ 86 Sr (t)(0.7029 至 0.7032)和放射性 ε 143 Nd (t)(+6.3 至 +8.2),与附近的钙碱性晚三叠世 Darran Suite I-相比土化侵入体的类型岩体。与全球深成岩 (δ 146/144 ) 相比,Longwood Suite 样品的稳定 Nd 同位素比率变化很大 (δ 146/144 Nd = 233 ppm)Nd = 44 ppm)并反映磷酸盐矿物的去除。总的来说,这些地球化学特征与角闪岩残渣的浅层(无石榴石)熔融产生花岗岩是一致的,我们从中推断出相对薄的岩石圈。Longwood 组的最大面积增加率为 43 km 2 /Ma,大大低于岩浆弧完全建立时随后的深成组。我们提出了一种岩石构造模型,即冈瓦纳大陆边缘地壳在构造上被二叠纪洋内岛弧地壳和地幔岩石圈压下,随后地幔岩石圈熔化生成了长木组的同位素原始辉长岩和长长闪长岩岩体。

更新日期:2021-10-06
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