ABSTRACT

Whereas north of 33°S Devonian calcalkaline magmatism is notably absent, a Devonian arc is well developed to the south of 33°S. The Carrizalito, Pampa de los Avestruces, and Papagayos plutons are located in the Frontal Cordillera between 34° and 35°S. U-Pb zircon SHRIMP and LA-MC-ICP-MS data from the plutons yield Early Devonian ages of 389 ± 3 and 409 ± 3 for Carrizalito, 413 ± 2 Ma for Pampa de los Avestruces, and 346 ± 3, 393 ± 5 and 412 ± 3 Ma for Papagayos. The younger zircon age from the Papagayos pluton suggests that the Devo nian magmatism continued during the Early Carboniferous. Carrizalito and Pampa de los Avestruces plutons display metaluminous to slightly peraluminous granodiorite composition, with SiO₂ ranging from 65.1 to 67.8 wt.% and from 67.9 to 69.7 wt.%, respectively, and define a magnesian calc-alkalic trend. The Papagayos pluton is metaluminous to slightly peraluminous and slightly more evolved in composition (felsic granodiorite to monzogranite), with a SiO₂ content that varies between 68.4 to 70.5 wt.%, defining a magnesian with ferroan enrichment and calc-alkalic to alkali-calcic trend. These geochemical characteristics are similar to those reported for the Carboniferous arc granitoids in the Frontal Cordillera and Western Sierras Pampeanas at 27°–30°S and the archetypical calc-alkaline granitoids of the Peruvian Coastal batholith, indicating an Early Devonian magmatic arc. Rare earth element patterns ([La/Yb]_N = 10–20) with mostly slight to moderate negative Eu anomalies (Eu/Eu* = 0.62–0.80) and geochemical ‘spider-diagrams’ are similar to arc granitoids. Combined whole-rock Nd and Sr isotope data from the largest Carrizalito and Pampa de los Avestruces plutons (εNd_t = −4.3 and −4.6; ⁸⁷Sr/⁸⁶Sr_t = 0.70704 and 0.70762; T_DM = ca. 1.5 Ga) together with in situ U–Pb and Hf isotope data from magmatic zircon (εHf_t ranging from −16.5 to −5.4; T_DM = ca. 1.7 Ga), suggest a common older continental lithosphere source. The presented data indicate tectonic segmentation in the pre-Andean margin controlling the presence or absence of Devonian subduction-related magmatism. This segmentation is best explained by changes in the tectonic plate configuration occurred in a long-lived convergent margin.

摘要

虽然 33°S 以北泥盆系钙碱性岩浆作用明显不存在，但 33°S 以南泥盆纪弧发育良好。Carrizalito、Pampa de los Avestruces 和 Papagayos plutons 位于南纬 34° 至 35° 之间的锋状山脉。来自岩体的 U-Pb 锆石 SHRIMP 和 LA-MC-ICP-MS 数据得出 Carrizalito 的早泥盆纪年龄为 389 ± 3 和 409 ± 3，Pampa de los Avestruces 为 413 ± 2 Ma，以及 346 ± 3、393 ± 5 Papagayos 为 412 ± 3 Ma。来自 Papagayos 岩体的较年轻的锆石年龄表明，泥盆纪岩浆作用在早石炭世期间继续存在。Carrizalito 和 Pampa de los Avestruces 岩体显示出金属铝到轻微过铝的花岗闪长岩成分，具有 SiO ₂分别为 65.1 至 67.8 wt.% 和 67.9 至 69.7 wt.%，并定义了镁钙碱趋势。Papagayos 岩体为金属铝质至略微过铝质，并且在成分上略微演化（长英质花岗闪长岩至二长花岗岩），SiO ₂含量在 68.4 至 70.5 wt.% 之间变化，定义了具有铁富集和钙碱至碱钙的镁质趋势。这些地球化学特征与报道的 27°–30°S 的 Frontal Cordillera 和 Western Sierras Pampeanas 的石炭纪弧花岗岩以及秘鲁沿海基岩的原型钙碱性花岗岩相似，表明早泥盆世岩浆弧。稀土元素模式（[La/Yb] _N = 10–20) 大部分为轻度至中度负 Eu 异常 (Eu/Eu* = 0.62–0.80) 和地球化学“蜘蛛图”类似于弧形花岗岩。来自最大的 Carrizalito 和 Pampa de los Avestruces 岩体的全岩 Nd 和 Sr 同位素数据（εNd _t  = -4.3 和 -4.6；⁸⁷ Sr/ ⁸⁶ Sr _t  = 0.70704 和 0.70762；T _DM  = ca. 1.5 Ga）与来自岩浆锆石的原位U-Pb 和 Hf 同位素数据（εHf _t范围从 -16.5 到 -5.4；T _DM = 大约 1.7 Ga），表明一个共同的较老的大陆岩石圈源。所呈现的数据表明前安第斯边缘的构造分割控制着泥盆纪俯冲相关岩浆活动的存在与否。这种分割最好的解释是在长期收敛边缘发生的构造板块配置的变化。