Upper Cretaceous intrusives in the Coastal Cordillera near Valdivia: forearc magmatism related to the passage of a triple junction?

IF 0.9 4区 地球科学 Q3 GEOLOGY
Denisse de la Fuente, O. Figueroa, D. Demaiffe, M. Mella, P. Duhart, D. Quiroz, J. Muñoz, V. Oliveros
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引用次数: 0

Abstract

Upper Cretaceous intrusives of limited extent crop out in the Coastal Cordillera near of Valdivia (39o48’ S), 100 km west of the main topographic divide of the Andean Cordillera. Given that plutonic rocks of the same age crop out at the same latitudes in the high Andes the coastal intrusives emplaced in a forearc position in the upper plate of a subduction setting. They correspond to hypabyssal intrusives displaying mainly porphyritic texture and lithological variations with microtonalites (minor), porphyritic microgranodiorites (main) and microgranites. They intrude the Upper Paleozoic-Triassic accretionary complex of the Bahia Mansa Metamorphic Complex. These intrusives, that comprise the Chaihuin Pluton and minor stocks of porphyritic felsic rocks, have calc-alkaline affinities with metaluminous and peraluminous character. They are geochemically similar to the contemporaneous main arc-related plutonic rocks of the Gualletue Plutonic Group. The microgranitoids and dacitic rocks from Los Boldos, the low and Loncoche are peculiar because they show an apparently adakitic affinity in Sr/Y and LaN/YbN discriminant diagrams; nevertheless Sr contents of these rocks (<<400 ppm) preclude a true adakitic character. The petrogenesis of the Chaihuin Pluton, is consistent with an evolution from tonalite to granite by successive fractional crystallization of plagioclase, amphibole, biotite, Fe-Ti oxides, apatite and zircon. The initial 87Sr/86Sr ratio (0.70411-0.70745), eNd (+4.24 to -3.09) and present-day Pb isotopic ratios (206Pb/204Pb: 18.616 to 18.708; 207Pb/204Pb: 15.620 to 15.635; 208Pb/204Pb: 38.573 to 38.662) of these rocks indicate that depleted mantle derived-magmas were contaminated by assimilation of crustal material at the base of the paleo accretionary prism or by subduction erosion. The heat required to explain mantle melting beneath the forearc crust could be supplied by the subduction of a young and hot oceanic slab and/or an active spreading ridge, as attested in similar settings in the world. According to plate reconstruction models the studied forearc intrusives would be generated during the migration of a triple junction that passed near Valdivia between 100 and 70 Ma.
瓦尔迪维亚附近海岸科迪勒拉的上白垩统侵入岩:与三联结通道有关的弧前岩浆活动?
在安第斯山脉主要地形分界线以西100公里处,瓦尔迪维亚(39o48’S)附近的海岸科迪勒拉(Coastal Cordillera)出现了范围有限的上白垩纪侵入物。考虑到在安第斯山脉的高海拔地区,同一年代的深成岩在同一纬度生长,海岸侵入物在俯冲背景下位于上板块的弧前位置。它们对应于以斑岩结构为主的浅浅侵入岩,岩性变异以微闪长岩(次要)、斑岩微花岗闪长岩(主要)和微花岗岩为主。它们侵入了巴伊亚-曼萨变质杂岩的上古生统-三叠纪增生杂岩。这些侵入岩由柴湖岩体和少量斑岩型长英质岩组成,具有钙碱性亲和性,具有铝质和过铝质特征。它们在地球化学上与古列特深成岩群同时期的主弧相关深成岩相似。在Sr/Y和LaN/YbN判别图中,Los Boldos、low和Loncoche的微花岗岩和英质岩石表现出明显的绿质亲和性,是其独特之处;然而,这些岩石的锶含量(<< 400ppm)排除了真正的埃达克岩特征。柴隐岩体的岩石成因符合斜长石、角闪孔、黑云母、铁钛氧化物、磷灰石和锆石的连续分馏结晶,从闪长岩到花岗岩的演化过程。初始87Sr/86Sr比值(0.70411-0.70745),eNd(+4.24 - -3.09)和现今Pb同位素比值(206Pb/204Pb: 18.616 - 18.708;207Pb/204Pb: 15.620 ~ 15.635;208Pb/204Pb: 38.573 ~ 38.662)表明枯竭的幔源岩浆受到了古增生棱柱底部地壳物质同化或俯冲侵蚀的污染。解释弧前地壳下地幔融化所需的热量可以由年轻而热的海洋板块和/或活跃的伸展脊的俯冲提供,这在世界上类似的环境中得到了证明。根据板块重建模型,所研究的弧前侵入将在100 ~ 70 Ma之间经过Valdivia附近的三结迁移过程中产生。
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来源期刊
Andean Geology
Andean Geology 地学-地质学
CiteScore
3.90
自引率
0.00%
发文量
17
审稿时长
>12 weeks
期刊介绍: This journal publishes original and review articles on geology and related sciences, in Spanish or English, in three issues a year (January, May and September). Articles or notes on major topics of broad interest in Earth Sciences dealing with the geology of South and Central America and Antarctica, and particularly of the Andes, are welcomed. The journal is interested in publishing thematic sets of papers and accepts articles dealing with systematic Paleontology only if their main focus is the chronostratigraphical, paleoecological and/or paleogeographical importance of the taxa described therein.
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