海洋地质与环境重点实验室知识库

华北陆块与扬子陆块是中国东部最大的两个陆块，拥有最大的两个前寒武纪基底。然而，关于华北陆块和扬子陆块的前寒武纪地壳演化仍存在争议。基于此，本文选取华北陆块南缘的新元古代罗圈组冰碛岩和扬子陆块北缘的新元古代南沱组冰碛岩作为研究对象，开展了系统的碎屑锆石U-Pb年代学、全岩主、微量元素和Sr同位素研究。结果表明，南沱组冰碛岩碎屑锆石年龄主要集中在新元古代850 Ma~700 Ma，峰值年龄为750 Ma，与扬子北缘的岩浆记录相对应。根据南沱组冰碛岩样品最年轻的碎屑锆石组的加权平均年龄得出南沱组冰碛岩的最大沉积年龄为约653 Ma，指出南沱冰期与全球冰期背景下的Marinoan冰期相对应。地球化学资料表明，南沱组冰碛岩的源岩具有弧型岩浆岩的地球化学特征，表现为富集大离子亲石元素（如：Rb、Ba和K）和亏损高场强元素（如：Nb、Ta和Ti）。结合扬子北缘广泛分布具有弧型地球化学特征的新元古代长英质和镁铁质岩体的地质资料，本研究认为扬子北缘的中新元古代岩浆岩形成于与俯冲相关的安第斯型活动大陆边缘环境（vs. 地幔柱/裂谷环境），并且相关的岩浆活动持续了超过150 Myrs（850~700 Ma）。与南沱组锆石年龄分布非常集中的情况不同，罗圈组冰碛岩的锆石年龄变化范围较大，主要分布于4个年龄区间：2700 Ma~2300 Ma，2130 Ma~2000 Ma，1900 Ma~1750 Ma和1700 Ma~1550 Ma。这些年龄大多与华北陆块的岩浆记录相对应，指出罗圈组冰碛岩沉积物的主要源区为华北陆块前寒武基底岩石。通过对罗圈组冰碛岩用全岩Rb-Sr等时线定年方法得出罗圈组冰碛岩的沉积年龄为约659 Ma。并且前人在杨坡剖面的东坡组（罗圈组的上覆地层）的红色白云岩碎屑中发现了Marinoan冰期盖层碳酸盐岩独特的沉积结构，说明罗圈冰期可能也与Marinoan冰期相对应。也就是说，我们在扬子陆块和华北陆块发现了同时期的新元古代冰川沉积，而在两个遥远的不同大陆上发现同时期的新元古代冰川沉积支持了“雪球地球”假说的基本预测。此外，在华北南缘和东缘大量格林威尔期岩浆锆石的发现，结合古地磁和岩墙群的文献资料，说明华北陆块是新元古代罗迪尼亚超大陆的一部分。

为了进一步认识华北陆块早期大陆地壳生长与演化，本论文还以泰山地区与TTG在空间上密切共生的新太古代闪长岩和角闪石岩为研究对象，探究其母岩浆性质和岩石成因。LA-ICP-MS锆石U-Pb定年结果显示，1件闪长岩和2件角闪石岩样品的结晶年龄分别为2615±8 Ma、2627±14 Ma和2616±10 Ma，与全岩-矿物Rb-Sr等时线定年（2621±7 Ma）得到的年龄结果一致，表明其均属于新太古代~2.6 Ga岩浆作用的产物。闪长岩和角闪石岩为同源岩浆不同演化阶段的产物，其中角闪石岩为早期堆晶。计算得到的与闪长岩和角闪石岩中角闪石平衡的熔体（即：母岩浆）具有低Mg^#值（平均值分别为31.9和43.4）、富集Ba、Pb等大离子亲石元素、亏损Nb、Ta、Zr、Ti等高场强元素，以及具有平坦型重稀土元素配分型式。此外，岩浆具有低的⁸⁷Sr/⁸⁶Sr初始值和接近亏损地幔的ɛ_Nd(t) (-0.01~2.87) 和ɛ_Hf(t) (-0.76~4.89)，且两阶段Nd模式年龄T_2DM为2.97~2.73 Ga。根据区域地质背景和上述地球化学特征，我们认为研究区闪长岩和角闪石岩的母岩浆可能来自于幔源岩浆底侵引起新生基性下地壳在角闪岩相条件下熔融形成安山质母岩浆。鲁西地区在2.9~2.7 Ga经历大规模大陆地壳生长，新太古代晚期2.6~2.5 Ga经历广泛大陆地壳生长和重熔，幔源岩浆的幕式底侵作用可能是华北陆块新太古代大陆地壳生长和分异的主要动力学原因之一。

The North China and Yangtze blocks are the two largest Precambrian basement blocks in continental China. Although many studies are available in the literature, the crustal evolution of the Yangtze and North China blocks in the Precambrian remains controversial. We have carried out a combined study of detrital zircon U-Pb dating, bulk-rock geochemistry and Sr isotope analysis for the Neoproterozoic tillites from the northern Yangtze Block (Nantuo Formation) and the southern North China Block (Luoquan Formation). The Nantuo tillite contains detrital zircons dominated by Neoproterozoic ages (850–700 Ma) with a prominent age peak at 750 Ma, corresponding to the known magmatic events in the northern Yangtze Block. The weighted mean ²⁰⁶Pb/²³⁸Pb age of the youngest zircon group is~653 Ma, providing the maximum depositional age for the Nantuo Formation, which confirms the Marinoan age of the Nantuo tillite. The geochemical data show that the source rocks of the Nantuo tillite have an arc geochemical signature with enrichment in LILEs (e.g., Rb, Ba and K) and depletion in HFSEs (e.g., Nb, Ta and Ti). These data, together with the regional geology, suggest that the late Neoproterozoic magmatism along the northern margin of the Yangtze Block represents an Andean-type active continental margin setting and the related magmatism lasted for more than 150 million years. Contrary to the very limited age variation of the zircons in the Nantuo Formation, the zircon ages of the Luoquan tillite exhibit a wide range. Broadly, the concordant zircons show four major groups: 2700–2300 Ma, 2130–2000 Ma, 1900–1750 Ma and 1700–1550 Ma, which correlates well with important tectono-magmatic events within the North China Block. Thus, we infer that the Precambrian basement of the North China Block was the most significant provenance of detrital zircons of the Luoquan tillite. The robust bulk-rock Rb-Sr isotope age of~659 Ma, together with the presence of well-known sedimentary structures of Marinoan cap carbonates in the red dolostone clasts, indicating that the Luoquan tillite is also of Marinoan age. The same glacier age recorded in the two remotely different blocks in the late Proterozoic provides supporting evidence for the existence of a Cryogenian Snowball Earth. In addition, many Grenvillian detrital zircons found in the southern and eastern North China Block may suggests a close relationship between the North China Block and the Rodinia supercontinent.

In order to further understand the crustal growth and reworking events in the evolution of the North China Block, we also carried out lithological, geochronological and geochemical analyses for the representative diorite and hornblendite in the Taishan area. Both diorite and hornblendite are of cumulate origin. Zircon U-Pb dating gives crystallization ages of 1 diorite and 2 hornblendite samples are 2615±8 Ma, 2627±14 Ma and 2616±10 Ma, respectively, which are consistent with the emplacement age of 2621±7 Ma obtained using bulk rock-mineral Rb-Sr isochron dating. They are all products of the Neoarchean ~2.6 Ga magmatism. The calculated parental magmas in equilibrium with amphibole in diorite and hornblendite are andesitic melt with Mg^# » 31.9 and 43.4, respectively, which is consistent with the hornblendite being of early cumulate origin and with the diorite being of cumulate origin from the parental magma that have undergone significant fractional crystallization dominated by plagioclase and hornblende. The calculated parental magmas are enrichment in large ion lithophile elements (LILEs; e.g., Ba, Pb), marked depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Zr and Ti), and flat heavy rare earth element patterns. In addition, the magma exhibit low initial ⁸⁷Sr/⁸⁶Sr (0.701555±12) and depleted ɛ_Nd(t) (-0.01~2.87) and ɛ_Hf(t) (-0.76~4.89) close to those of depleted mantle, giving a two-stage Nd model age T_2DM is 2.97~2.73 Ga. On the basis of these data and in the context of the regional geology, we infer that the regional must have undergone large scale underplating of mantle derived basaltic magmas that have caused deep crustal melting, granitoid magmatism and crustal growth in this part of the North China Block during ca. 2.9~2.7 Ga, and extensive crustal growth and remelting at ca. 2.6~2.5 Ga.