The 595–560 Ma post-collisional magmatism in the Alto Pajeú Domain, Borborema Province, northeastern Brazil, is characterized by the intrusion of voluminous high-K calc-alkaline, shoshonitic and ultrapotassic magmas. The nature of the source rocks and the geodynamic context associated with the generation and emplacement of this post-collisional magmatism is still enigmatic. In this study, we report new UPb ages, elemental geochemistry, mineral chemistry and SrNd isotope data for rocks from the Pajeú batholith. LA-ICP-MS zircon UPb dating revealed that this batholith was formed by at least two distinct magma pulses. The oldest pulse (ca. 592 Ma) consists mainly of porphyritic quartz monzonites to monzogranites with abundant mafic microgranular enclaves (MMEs), while the younger pulse (ca. 568 Ma) is composed of equigranular biotite granites. The porphyritic monzogranites are magnesian, have intermediate to acidic compositions (SiO = 62.2–67.6 wt%) and high Mg# (44–55) values. In contrast, biotite granites are ferroan, have high SiO (69.3–73.1 wt%) and low Mg# (16–36) values. Both rock types are characterized by enrichment in LREE and LILE and depletion in HREE and HFSE, but with more fractionated REE patterns for biotite granites ([La/Yb] = 67.4–101.5) than for porphyritic rocks ([La/Yb] = 35.8–54.8). The SrNd isotopic values for the different rock types are roughly similar. However, biotite granites show slightly higher initial Sr/Sr (0.71265–0.71412) ratios and more negative (t) (−18.45 to −18.67) values than porphyritic monzogranites (initial Sr/Sr ratios from 0.71077 to 0.71155 and (t) from −16.04 to −16.96). These geochemical and isotopic signatures, together with mineral chemistry data, suggest different sources for the two rock types. The biotite granites are of purely crustal origin, derived by partial melting of a Paleoproterozoic amphibolitic lower continental crust, while a mixture of melts derived from the Paleoproterozoic continental crust and magmas derived from an enriched lithospheric mantle could explain the origin of porphyritic monzogranites. Based on the spatial and temporal distribution of magmatic rocks from the Alto Pajeú Domain, we suggest that a local lithospheric thinning model could explain the evolution of post-collisional magmatism in this region of the Borborema Province.

中文翻译：

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巴西东北部新元古代阿尔托帕杰乌域的碰撞后岩浆作用和局部岩石圈减薄：地球化学和同位素证据

巴西东北部 Borborema 省 Alto Pajeú 域的 595–560 Ma 碰撞后岩浆活动的特点是大量高钾钙碱性、钾闪质和超钾质岩浆侵入。烃源岩的性质以及与碰撞后岩浆作用的产生和就位相关的地球动力学背景仍然是个谜。在这项研究中，我们报告了帕杰乌岩基岩石的新 UPb 年龄、元素地球化学、矿物化学和 SrNd 同位素数据。 LA-ICP-MS 锆石 UPb 定年表明，该岩基是由至少两次不同的岩浆脉冲形成的。最古老的脉冲（约 592 Ma）主要由斑状石英二长岩到二长花岗岩组成，具有丰富的镁铁质微粒包体（MME），而较年轻的脉冲（约 568 Ma）则由等粒黑云母花岗岩组成。斑状二长花岗岩为镁质，具有中酸性成分 (SiO = 62.2–67.6 wt%) 和高 Mg# (44–55) 值。相比之下，黑云母花岗岩为铁质，具有高 SiO (69.3–73.1 wt%) 和低 Mg# (16–36) 值。两种岩石类型的特征都是轻稀土元素和轻稀土元素富集，重稀土元素和高频率稀土元素贫化，但黑云母花岗岩 ([La/Yb] = 67.4–101.5) 的稀土元素分馏模式比斑岩岩石 ([La/Yb] = 35.8) 更多。 –54.8）。不同岩石类型的 SrNd 同位素值大致相似。然而，黑云母花岗岩的初始 Sr/Sr 比 (0.71265–0.71412) 比斑状二花岗岩略高，(t) 值更负（−18.45 至 -18.67）（初始 Sr/Sr 比为 0.71077 至 0.71155，(t) 为 - 16.04 至 -16.96)。这些地球化学和同位素特征以及矿物化学数据表明这两种岩石类型的不同来源。黑云母花岗岩纯粹是地壳成因，是由古元古代角闪岩下大陆地壳部分熔融而形成的，而源自古元古代大陆地壳的熔体和源自富集岩石圈地幔的岩浆的混合物可以解释斑状二长花岗岩的起源。根据 Alto Pajeú 域岩浆岩的时空分布，我们认为局部岩石圈减薄模型可以解释 Borborema 省该地区碰撞后岩浆作用的演化。