摘要
近些年来华北克拉通在21～17亿年前发生的地质事件很受关注。我们的研究表明,在这个时期华北克拉通曾发生了一系列重大事件,它们不是简单的一个造山或裂谷事件,应通过对地质事件群的厘定,分解为21～19亿的造山和18.5～17亿的裂解两个事件亚群21～19亿造山事件表现为:(1) 在新太古代末华北克拉通上发育的一些古元古代活动带;(2) 强烈的褶皱变形和绿片岩-绿帘角闪岩相的变质作用;(3) 富钾质的花岗岩席侵入;(4) 镁铁质岩墙侵位于活动带中。该事件以晋豫和辽河活动带为代表,或许与前罗迪尼亚(哥伦比亚)造山有关,它们应结束于～1930Ma前后。18.5～17亿年裂解事件与前者不属同一构造体制,主要表现为:(1) 地幔大规模上涌,下部地壳整体抬升至地表,伴随强烈的混合岩化和网格状韧性变形,面状分布的麻粒岩相-角闪岩相岩石在中生代才受到较强烈的改造;(2) 在华北多处出露的高压麻粒岩岩墙的中压麻粒岩相和角闪岩相退变质作用,时代为1820～1800Ma和1790～1760Ma,P-T-t轨迹为等温降压和降温降压型,少量有自变质结构,此类高压麻粒岩-麻粒岩岩墙不具大陆碰撞变质的特征;(3) 基底抬升被裂谷事件紧随,熊耳-安沟-中条裂谷和燕山裂谷可能属同一体系,熊耳群火山岩最早的喷发为1917Ma,顶界年龄为1760Ma;(4) 古老地幔从2000Ma的亏损在1900～ 1760Ma快速变为富集。由此推测18亿年事件的实质是在地幔规模性上隆引导下,基底整体抬升,而后紧随陆内裂陷槽发育。在时代与成因上与早-中元古代超大陆裂解事件相关联。镁铁质岩墙广泛出露于克拉通内,以前的研究多认为它们与裂谷有关。新的研究表明,岩墙有变质和几乎未变质两种。变质的岩墙有2150Ma的结晶年龄,和1930～1970Ma(高压麻粒岩相)以及1830Ma(麻粒岩相)变质年龄,而未变质的岩墙有1780～1750Ma的结晶年龄,与变质岩墙的
Recently, the geological events and their significance during period of ca 2.1～1.7 Ca are attracted as an interesting study focus. Our study shows that a series of geological evenls occurred in the Norlh China cralon at that time do not belong to a simple orogenic or rifting tectonic process. These events could be divided into two event-suhgroups, i.e. 2.1～1.9 Ga orogenic event subgroup and 1.85～1.7 Ga rift event-subgroup. The major indications of the 2.1～1.9 Ga event-subgroup are listed below:(1) several mohile behs developed within the Neoarchean craton; (2) strong folded deformation and metamorphism of greenschist-epidote amphibolite facies; (3) magmatic intrusion of K-granite sills and Mafic dyke swarm. This event-subgroup is represented by the Jinyu and Liaohe mobile belts, and was possihly related to the Pre-Rodinia (Columbia) orogeny. This event-subgroup ended at ca 1.93 Ga. The 1.85～1.7 Ga event-subgroup is a rifting event. Its major indications are: (1) mantle upwelling and lower crust uplifting to the surface at whole, accompanied by strong migmatization and ductile deformation; (2) high-pressure granulites underwent moderatepressure granulite-amphibolite facies overprint with 1820～1800 Ma and 1790～1760 Ma metamorphic ages and decompressional P-T path; (3) rifting event closely followed the basement uplifting, and the Xionger-Angou-Zhongtiao rift and Yanshan rift are of a same rift regime: (4) old mantle rapidly became richer at ca 1900～1760 Ma from a depleted composition. Therefore, we suggest that the ～1.8 Ga event-subgroup represents a continental geological process within craton: an upwelling mantle caused the craton basement uplifting at whole and closely followed by an aulacogen system. Our new study shows that mafic dykes can be divided into two types: metamorphosed and unmetamorphosed. The former has 2150 Ma forming age and 1970～1930 Ma (high-pressure granulite facies) and 1830 Ma (granulite facies) metamorphic ages. The later has 1780～1750 Ma forming age which is similar t
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2004年第6期1343-1354,共12页
Acta Petrologica Sinica
基金
基金重点项目(40234050)
中科院创新项目(KZCX1-07)资助
