冀东地区新太古代麻粒岩相变质作用及其大地构造意义

Neoarchean granulite facies metamorphism and its tectonic implications from the East Hebei terrane

冀东地区的早前寒武纪基底保留有太古宙克拉通普遍发育的"穹窿-龙骨构造",如卢龙-双山子表壳岩系呈近南北向带状分布于以TTG片麻岩为主构成的太平寨卵形构造域、迁安片麻岩穹窿和安子岭片麻岩穹窿之间,洒河桥线性构造带呈北东向切割太古宙构造线分布。在太平寨卵形域和洒河桥线性带中常见有零散分布的麻粒岩相表壳岩块体和古元古代变质基性岩墙。表壳岩块体包括基性和泥砂质麻粒岩和BIF型铁矿等,变质基性岩墙也发育高压麻粒岩相组合,变质锆石年龄为～1.81Ga。本文总结了近年来对该区麻粒岩的研究进展。太平寨卵形域中的基性麻粒岩以中粒二辉麻粒岩为主,有些样品的角闪石周围出现叠加变质形成的微粒矿物组合,个别样品见辉石周围发育石榴石冠状体,形成‘红眼圈’结构;利用稀土元素温度计确定二辉麻粒岩的峰期达到了～1000℃的超高温条件,麻粒岩中的锆石仅记录新太古代末期变质年龄(～2.50Ga),与周围TTG质岩石的结晶时间近于相同。洒河桥线性带中的基性麻粒岩以细粒高压麻粒岩组合为主,偶见中粒二辉麻粒岩残留,其峰期P-T条件分别为800～860℃/1.0～1.2GPa和950～1070℃/1.0GPa;麻粒岩中锆石主体记录新太古代末期变质年龄,但出现少量古元古代变质锆石(1.97～1.83Ga),石榴石-全岩Lu-Hf等时线年龄为1.77～1.78Ga。由此推测太平寨和洒河桥地区都经历了新太古代末期超高温麻粒岩相变质作用,又在古元古代晚期遭受了高压麻粒岩相差异性叠加,太平寨地区受叠加较弱,仍然保留太古宙的卵形构造,而洒河桥一带受叠加较强,形成了线性变形带。太平寨卵形域的泥砂质麻粒岩可识别出4阶段的变质矿物组合:包括包体组合、峰期组合、固相线(或最后)组合以及叠加组合。相平衡模拟表明固相线组合的P-T条件为870～890℃/～0.7GPa,峰期组合可达到1000℃/1.1GPa,石榴石中富钙斜长石包体指示麻粒岩早期经历了低压高温变质阶段。由此构建麻粒岩P-T轨迹为逆时针型,包括3个阶段:低压加热至超高温(AG-I),近等温升压至压力峰期(～1.1GPa)(AG-II),和峰后降压降温至固相线(AG-IIIa)以及在亚固相线下的降温降压过程(AG-IIIb)。锆石定年表明泥砂质麻粒岩原岩沉积年龄稍早于2.50Ga,变质年龄为2.48～2.50Ga。泥砂质麻粒岩的峰期变质条件和时代均与二辉麻粒岩一致,叠加组合的P-T条件与高压麻粒岩相似,反映泥砂质麻粒岩也受到了古元古代晚期变质作用影响。依据太平寨麻粒岩的逆时针型P-T轨迹,推测麻粒岩相变质作用包括如下构造过程:(i)AG-I指示表壳岩层受到后续地幔极高温岩浆喷发被埋深加热,或者受到下部TTG质岩浆海的加热过程;(ii)AG-II指示被加热的岩石(总伴有BIF铁矿层)被破碎并在在密度驱动下沉入岩浆海深部,达到下地壳深度;(iii)AG-IIIa/b对应沉入岩浆海深部的岩石伴随穹窿上升发生减压冷却的过程。很多地质观测和数值模拟研究表明太古宙克拉通的形成受太古宙特有的垂直构造体制控制,与太古代之后线性造山带的构造体制完全不同。

The Early Precambrian basement in the East Hebei terrane is characterized by the occurrence of ‘dome and keel structures＇ like those in other Archean cratons. For instance,the Lulong-Shuangshanzi supracrustal rock series are distributed as a nearly north-south belt among the Taipingzhai ovate-structural domain,the Qian＇an gneiss dome and the Anziling gneiss dome,that are comprised dominantly of TTG gneisses. These units were cut by the Saheqiao linear structural belt extending in north-east. In the Taipingzhai ovate-structural domain and Saheqiao linear belt commonly occur numerous enclaves or slivers of supracrustal rocks and Paleoproterozoic metabasic dykes. The slivers of supracrustal rocks include basic and pelitic-psammitic granulites interlayered with BIF,and the metabasic dykes were subjected to high-pressure granulite facies metamorphism with zircon ages at - 1. 81 Ga. This article provides a review of the recent progresses in studying the granulites in this area. Basic granulites in the Taipingzhai ovate-structural domain are mainly of medium-grained two-pyroxene granulites,but some samples show the overprinting assemblages consisting of tiny vermicular grains of minerals around amphibole,while a few samples exhibit garnet coronas around pyroxene,forming a ‘red-eye socket＇texture. These basic granulites were estimated to reach ultrahigh temperature of - 1000℃ at the peak stage using two-pyroxene REE thermometer,and zircons were dated to only record the Neoarchean metamorphic ages at - 2. 5Ga,almost the same as the crystallization time of the surrounding TTG rocks. Basic granulites in the Saheqiao linear belt are dominated by fine-grained highpressure granulite assemblages with two-pyroxene assemblages occasionally survived. Their peak P-T conditions were estimated to be800 - 860℃/1. 0 - 1. 1GPa and 950 - 1070℃/1. 0GPa,respectively. Zircons in these granulites were dated to dominantly record the Neoarchean metamorphic ages,but a small amount of Paleoproterozoic metamorphic zircons of 1. 97 - 1. 83 Ga are present. For the high-pressure granulite assemblages,garnet-whole rock Lu-Hf isochron yields ages of 1. 77 - 1. 78 Ga. As a result,it is inferred that the Taipingzhai and Saheqiao areas both experienced the ultrahigh temperature granulite facies metamorphism in the Late Neoarchean,and they suffered the different overprinting of high-pressure granulite facies in the Late Paleoproterozoic. The Taipingzhai area was weakly affected by the overprinting event,and thus,retained the Archean oval structures,whereas the Saheqiao belt was strongly overprinted,forming a linear deformation zone. Pelitic-psammitic granulites in the Taipingzhai ovate-structural domain were identified to have four stages of metamorphic mineral assemblages of the inclusion,peak,solidus（ or final） and overprinting. Phase equilibria modelling shows the P-T conditions of 870 - 890℃/- 0. 7GPa for the solidus assemblage and - 1000℃/1. 1GPa for the peak assemblage,while the calcium-rich plagioclase inclusions in garnet was modelled to indicate that the granulites experienced an early metamorphic stage at low-pressure and high-temperature conditions. As a consequence,an anticlockwise P-T path was constructed for the granulites,including three stages： heating under low-pressure to the ultrahigh temperature stage（ AG-I）, near-isothermal compression to the peak stage（ - 1. 1GPa）（ AG-II）,and the subsequent cooling with decompression to the solidus（ AG-IIIa） and the subsolidus cooling with decompression stage（ AG-IIIb）. Zircon dating indicates that the deposition age of the protoliths of the peliticpsammitic granulites was slightly earlier than 2. 50 Ga and the metamorphic age was 2. 48 - 2. 50 Ga. The peak metamorphic conditions and age of the pelitic-psammitic granulites are consistent with those of the two-pyroxene granulites,whereas their overprinting assemblage is similar to the basic high-pressure granulites in P-T conditions,reflecting that the pelitic-psammitic granulites were also affected by the late Paleoproterozoic metamorphic event. Based on the anticlockwise P-T path of the Taipingzhai granulites,it is speculated that the Neoarchean granulite facies metamorphism includes the following tectonic processes：（ i） the AG-I stage indicates that supracrustal layers were heated by the subsequent eruptions of mantle-derived very high temperature magma,or by the underlying TTG magma ocean;（ ii） the AG-II stage suggests that the heated rocks were fragmented and dropped into the lower part of the magma ocean（ to the depth of the lower crust） driven by density（ the granulites always interlayered with BIF）;（ iii） and the AG-IIIa/b stage corresponds to the exhumation of the supracrustal rocks sunk into the deep magma ocean in response to the doming processes. Many geological observations and numerical simulations suggest that the formation of Archean cratons is controlled by the Archean unique vertical tectonics,which is completely different from that occurred in the post-Archean linear orogenic belts.