Protolith age and exhumation history of metagranites from the Dabie UHP metamorphic belt in east-central China: A multi-chronological study

Abstract

Metagranites (granitic orthogneisses) constitute one of the important lithologic units in the Dabie ultrahigh pressure metamorphic belt. They often display direct contact with paragneisses and eclogites. TIMS zircon U-Pb dating on the metagranites reveal that their protolith was crystallized as granitoids in Neoproterozoic ages of 755 to 611 Ma and thus correspond to rift magmatism associated with breakup of the Rodinia supercontinent in the northern margin of the Yangtze Block. Zircon U-Pb age and Ar-Ar ages of amphibole and biotite indicate that the granitoids experienced eclogite-facies metamorphism in Early Mesozoic with rapid exhumation at cooling rates of 8.7 to 10.2°C/Ma from 225 ± 6 to 181 ± 3 Ma in response to amphibolite-facies retrogression. Fission track ages of sphene, zircon and apatite indicate that the metagranites experienced slow exhumation with cooling rates of 0.88 to 1.25°C/Ma in Late Mesozoic. It is possible that in the first period of exhumation, the metagranites were uplifted from the mantle depth to lower crustal level at ca. 225 Ma, then rapidly reached middle to upper crustal levels between 204 Ma and 180 Ma. After the intensive magmatism in Early Cretaceous, the Dabie terrane was uplifted at the slow rate in response to unroofing. Either the rapid exhumation in the Early Mesozoic or the slow uplift in the Late Mesozoic in the Dabie terrane were contemporaneous with two rapid subsidence and sedimentation of the Hefei Basin in Early Jurassic and Paleogene, respectively. This suggests that the orogeny and basin-forming events were closely related to each other rather than independently separated.Keywords: metagranite, geochronology, cooling history, eclogite-facies, amphibolite-facies to the exact timing of UHP metamorphic event at either Early-middle Triassic or Late Triassic.In the Dabie UHP metamorphic belt, granitic orthogneisses are widespread Cong, 1996), which is termed as metagranites in this paper following the nomenclature of Xu et al. (1998). It was suggested that these metagranites did not experience the UHP metamorphism because no UHP index mineral was found until 2001. However, this suggestion was questioned by Carswell et al. (2000), who deduced from mineralogy and mineral chemistry that the metagranites were metamorphosed at P-T conditions of 36 to 18 kbar and 750 to 690°C. Furthermore, coesite inclusions were observed in zircons by Liu, F-L. et al. (2001) and Liu, J-B. et al. (2001), demonstrating that the metagranites did experience the UHP metamorphism.With respect to ages of metagranite protolith and UHP event in the Dabie terrane, a number of zircon U-Pb dating was carried out by the TIMS technique. The results show discordia upper and lower intercept ages of 800 to

Geochronology and geochemistry of gneissic metagranites in eastern Dabie Mountains: Implications for the Neoproterozoic tectono-magmatism along the northeastern margin of the Yangtze Block

“…For example, Rowley et al [3] reported an upper intercept age of 805 ± 15 Ma for gneissic metagranites at Shima, and Chavagnac et al [14] gave an upper intercept age of 720 ± 26 Ma for those at Bixiling, and considered them to be the protolith ages. Wang et al [15] reported several sets of upper and lower intercept ages for different metagranite plutons, such as 726 ± 15 Ma and 224.5 ± 6.3 Ma for Shuanghe, 714 ± 14 Ma and 209 ± 14 Ma for Bixiling, 720 ± 61 Ma and 197 ± 21 Ma for Xindian, and 755 ± 34 Ma and 299 ± 33 Ma for Huangzhen, and considered the upper and lower intercept ages to be the protolith ages and the subsequent UHP and/or retrograde metamorphic ages, respectively. The above dating results suggest that the protoliths of the gneissic metagranites in Dabie region were mainly formed in Neoproterozoic (700-800 Ma) [16].…”

“…The quartz grains are anhedral and display apparent wavy extinction, and some of them occur as lenticular poly-crystal aggregations. Plagioclases in Huangzhen, Daba, Shima and Shuanghe gneissic metagranites are mainly albite (An = 1-10), whereas those in Sanzusi pluton are albite-oligoclase (An = [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. They are all euhedral to subhedral, and lath in shape, with some grains showing inhomogeneous extinction due to metamorphism.…”

大别山东缘片麻状变质花岗岩的锆石U-Pb 年代学与地球化学: 对扬子板块北东缘新元古构造-岩浆作用的启示

“…对区内片麻状变质花岗岩的年龄前人主要运用 ID-TIMS 方法测定, 如 Rowley 等 [3] 测得石马片麻状 变质花岗岩的上交点年龄为(805±15) Ma, Chavagnac 等 [14] 测得碧溪岭片麻状变质花岗岩的上交点年龄为 (720±26) Ma, 并认为它们应代表原岩的形成年龄. Wang 等 [15] [16] , 但 由于 ID-TIMS 方法无法进行原位微区分析, 对于具 有复杂历史的锆石样品会得出混合年龄, 从而导致 无意义的定年结果.…”

“…他认为 A 型花岗岩 可进一步区分为构造环境及来源不同的 A 1 和 A 2 两组, A 1 组以其 Y/Nb 和 Y/Ta 比值与洋岛玄武岩相似为特 征, 岩浆起源于地幔并产于大陆裂谷或地幔热柱、热 点环境; A 2 组其比值则可由类似平均地壳变化至类 似岛弧玄武岩, 岩浆起源于地壳或由岛弧岩浆作用 派生, 产于后碰撞或后造山的张性构造环境. 在 Eby 提出的 A 型花岗岩亚类判别图解上, 原岩具有 A 型 [15] 花岗岩特性的第一组片麻状花岗岩均投影在 A 2 区(图 9, 10), 显示与岛弧玄武岩(IAB)的地球化学特征具亲 缘性. 结合在相对原始地幔标准化的蛛网图上, Sr, P, Ti, Nb 和 Ta 等元素呈显著的亏损特征, 表明这些片 麻状变质花岗岩的源区可能与俯冲作用有密切的联 系, 其原岩应形成于后碰撞或后造山的伸展引张环 境.…”