Continental flood basalts of the Huashan Group,northern margin of the Yangtze block – implications for the breakup of Rodinia

It is generally accepted that Neoproterozoic extension and dispersal of the supercontinent Rodinia was associated with mantle plume or superplume activities. However, plume-generated contemporaneous continental flood basalts (CFBs) have rarely been identified. In this study, we present geochronological and geochemical evidence for the basalts from the Liufangzui Formation of the Huashan Group in the Dahongshan region of east-central China. A representative sample yields a SHRIMP U–Pb zircon age of 824 ± 9 Ma, interpreted as the crystallization age of the rocks. Geochemically, these basalts belong to the subalkaline tholeiite series and display slight enrichments in light rare earth elements (LREE) and varying degrees of deficiency of high field strength elements (HFSE) such as Nb, Ta, and Ti. This pattern is very similar to that of CFBs from the Bikou Group and Tiechuanshan Formation in the northwestern Yangtze block in China and Siberia in Russia. The basaltic magmas underwent partially-fractional crystallization during ascent, but were not intensely influenced by crustal contamination. The characteristic element ratios and negative Hf isotopic analyses (?_Hf(t) = ?6.6–2.6) in zircons indicate that the parental magmas of the basalts might have been derived from an enriched lithospheric mantle rather than from the depleted mantle such as normal mid-ocean ridge basalts (N-MORBs). The geochemical signatures and regional geological characteristics show that these basalts were formed along intraplate continental rifts rather than in island arcs or ocean basins. Considering the coeval basic volcanic rocks in South China, we propose that these Huashan Group basalts represent the remnants of plume-generated CFBs and have close spatiotemporal ties with a coeval basic igneous province in Australia. Our results support the Neoproterozoic location of the South China block adjacent to southeastern Australia in the reconstruction model of the supercontinent Rodinia.     

北大巴山凤凰山岩体锆石U Pb LA ICP MS年龄及其构造意义

北大巴山凤凰山岩体是扬子陆块北缘典型的新元古代侵入岩群,对采自岩体的6个花岗岩(闪长岩)样品进行了LA-ICP-MS锆石U-Pb同位素定年分析。定年数据显示,岩浆活动可分为早期(797±6Ma)、中期(770±6Ma、774±5Ma)、晚期(755±6Ma、750±13Ma和743±6Ma)三个阶段,证实凤凰山岩体为多期岩浆侵入形成。这几期岩浆活动在扬子陆块具同步性和普遍性,广泛被记录在黄陵、汉南等其它新元古代杂岩中。凤凰山花岗岩完整记录了扬子陆块北缘南华纪初期的区域拉张—裂陷事件,其形成可能与导致Rodinia超大陆裂解作用的幕式地幔柱活动有关。本次研究对认识扬子陆块新元古代地壳生长与再造过程及Rodinia超大陆裂解机制具有重要意义。     

浙江次坞地区晋宁晚期双峰式岩浆杂岩带的地球化学特征：Rodinia超大陆裂解的岩石学记录

次坞地区晋宁晚期火山岩与侵入岩由基性岩（上墅组下段基性火山岩与次坞辉绿岩体）和酸性岩（上墅组上段酸性火山岩与道林山碱长花岗岩体）组成,它们在时空上紧密伴生,成因上密切相关,SiO₂ 含量集中分布在两个区间,缺乏SiO₂为57％~68％的中性及中酸性成分,构成一套双峰式岩浆杂岩组合。酸性岩在化学组成上富硅碱而贫钙镁,准铝-过铝质,富含Zr、Nb和稀土元素,FeO^*/MgO、Rb/Sr比值大,显示出A型花岗质岩石的成分特征;基性岩具有富铝、钠、钛和偏碱性的特征,形成于陆内至陆缘张裂环境。基性岩与酸性岩由不同的源区熔融形成,基性岩起源于亏损程度较低的地幔或来源于亏损地幔的岩浆受到陆壳物质的混染,酸性岩可能起源于地壳中既含有基性地壳组分和又含有酸性地壳组分的源区的部分熔融。双峰式岩浆杂岩的形成,是华南Rodinia超大陆裂解事件的岩石学记录,揭示了华南Rodinia超大陆裂解始于青白口纪。     

Rodinia超大陆裂谷裂陷高潮与裂-坳转换:来自塔里木西北缘阿克苏地区碎屑锆石的制约

阿克苏地区位于塔里木地块西北缘,是研究Rodinia超大陆裂谷裂解过程的关键地区.在阿克苏地区裂谷序列中,发育有不整合面.通过对阿克苏地区巧恩布拉克组、尤尔美那克组和苏盖特布拉克组碎屑锆石的LA-ICP-MS U-Pb年龄测定,对Rodinia超大陆裂谷期不整合面的形成时间、不整合面上下地层的物源变化以及不整合面的构造意义进行了讨论.碎屑锆石测年结果显示:尤尔美那克组于708 Ma后开始沉积,苏盖特布拉克组于577 Ma后开始沉积,被剥蚀的阿克苏群为尤尔美那克组和苏盖特布拉克组提供了局部物源.结合前人资料分析表明:①巧恩布拉克组与尤尔美那克组之间角度不整合面形成于717～708 Ma之间,苏盖特布拉克组与下伏地层间不整合面形成于580～577 Ma之间;②阿克苏地区724～717 Ma之间发生了一次幕式裂陷作用,这次幕式裂陷作用可能代表了 Rodinia超大陆裂谷裂陷期的高潮;③苏盖特布拉克组与下伏地层间不整合面是Rodinia超大陆裂谷裂-坳转换界面,这个界面时限为580～577 Ma;④"雪球地球"事件显著地促进了不整合面形成时的剥蚀作用,巧恩布拉克组与尤尔美那克组之间的角度不整合面、苏盖特布拉克组与下伏地层之间的不整合面的形成分别受到了 Sturtian冰期、Gaskiers冰期的影响.在本文中,不仅阿克苏地区裂谷期不整合面的成因机制得到了探讨,也一并完善了阿克苏地区裂谷的演化模式.     

扬子西缘新元古代裂谷盆地演化的年代学新证据及其意义

扬子西缘康滇裂谷是华南新元古代裂谷系的主要组成部分,对理解华南新元古代裂谷盆地演化与Rodinia超大陆裂解的响应关系具有重要意义。澄江组是康滇裂谷最为典型的沉积充填序列,其时代的准确限定是解析Rodinia超大陆裂解背景下盆地演化的重要前提。本文对滇东北巧家谓姑地区澄江组火山岩开展了LA-ICP-MS锆石U-Pb同位素年代学分析,获得澄江组底部玄武岩年龄806.4±6.7 Ma和下部凝灰岩年龄788.4±5.9 Ma。结合已有年龄数据,将澄江组沉积时代准确限定为800～720 Ma,与开建桥组及陆良组上部为同期异相产物。康滇裂谷的三个次级盆地演化基本同步,均于800 Ma左右开始全面接受沉积,是Rodinia超大陆的裂解的响应。     

华夏地块新元古代沉积记录与超大陆重建

新元古代-早古生代,Rodinia超大陆裂解至Gondwana大陆聚合的过程对华南构造格局与古地理演化具有重要的制约作用,然而在这过程中华夏地块的位置、华夏与扬子之间的关系等问题仍存在争议。本文通过同位素地层年代学、新元古代冰期事件及成矿事件对比了华夏地块武夷南部-南岭地区及华夏西缘新元古代中-晚期地层。在此基础上,对新元古代沉积物进行碎屑锆石物源分析,发现华夏西缘拉伸纪晚期-埃迪卡拉纪曾出现多次物源方向的转换,并在成冰纪晚期同时出现来自华南内部和外部的双向物源。结合浅海陆棚的沉积环境,表明此时扬子与华夏之间并没有宽阔的大洋相隔。华夏地块拉伸纪晚期-埃迪卡拉纪碎屑岩中包含了大量中元古代至新元古代早期的碎屑锆石,与东冈瓦纳北部印度、东南极等地区的沉积记录吻合,指示华夏地块至少自750 Ma时期就接收到来自印度北缘的碎屑物质。在Rodinia超大陆裂解至Gondwana大陆聚合期间,华南一直保持在超大陆边缘的古地理位置,与印度北缘相连。     

A Laurentian affinity for the Embu Terrane,Ribeira Belt (SE Brazil), revealed by zircon provenance statistical analysis

New and compiled detrital zircon U–Pb ages from the southern Neoproterozoic–Cambrian Ribeira Belt, SE Brazil, demonstrate Laurentian affinity of the Embu Terrane which is statistically distinct from the adjoining Apiaí and São Roque terranes with cratonic affinity (e.g., São Francisco Craton). Zircon provenance results indicate that the type-area of the Embu Terrane is dominated by detrital zircon age modes at ca. 1200 Ma, 1400 Ma, and 1800 Ma, with maximum depositional age of ca. 1000 Ma. In contrast, the Apiaí and São Roque terranes are dominated by Paleoproterozoic detrital zircon ages (ca. 2200–2000 Ma age dominant component), with maximum depositional ages of ca. 1400 Ma and 1750 Ma, respectively. Multidimensional scaling (MDS) analysis of non-parametric similarity measurements on zircon age populations indicates for the first time that the Embu Terrane encompass two statistically distinct detrital zircon age spectra, which is also reflected in the metamorphic zircon age record. The statistical characterization of the Embu Terrane through populational metrics allow a quantitative comparison with surrounding tectonic domains and rock samples classified such as Embu-type. Our results clearly highlight the distinction between the statistically differentiated Embu Terrane from the Apiaí and São Roque terranes, supporting an allochthonous interpretation. In addition, we demonstrate that rocks samples previously classified as Embu-type are significantly dissimilar to the definition of Embu Terrane, failing to support alternative tectonic models (e.g., intracontinental evolution). Detrital zircon age spectra reveal that the Apiaí and São Roque terranes have similar zircon provenance to domains sourced from the São Francisco Craton, whereas detrital zircon populations from the Embu Terrane have greater affinity with SW Laurentia basins (and their inferred sediment sources), consistent with previous findings. Therefore, we interpret the Embu Terrane as a Rodinia descendant developed along the active margin of the SW Laurentia that collided with the Ribeira Belt during early Neoproterozoic (810–760 Ma).     

Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block,China

The Neoproterozoic Sugetbrak Formation in the Aksu area, which is located at the northwest margin of Tarim Block, comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent. However, the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear. In this study, the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some detail along three sections. The authors found that the mafic rocks in these three typical sections were mainly composed of pyroxene and plagioclase, containing a small amount of Fe-Ti oxides and with typical diabasic textures. All the mafic rocks in this region also showed similar geochemical compositions. They were characterised by high TiO₂ contents (1.47% –3.59%) and low MgO (3.52% –7.88%), K₂O (0.12% –1.21%). Large ionic lithophile elements (LILEs) (Rb, Sr, and Cs) were significantly depleted. Meanwhile, high field strength elements (HFSEs) were relatively enriched. In the samples, light rare earth elements (LREEs) were enriched, while heavy rare earth elements (HREEs) were depleted. Based on the Zr/Nb, Nb/Y, and Zr/TiO₂ ratios, the Aksu mafic rocks belong to a series of sub-alkaline and alkaline transitional rocks. The mafic rocks along the three typical sections showed similar initial values of ⁸⁷Sr/⁸⁶Sr (I_Sr) (0.7052–0.7097) and ε_Nd(t) (–0.70 to –5.35), while the Pb isotopic compositions with ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of 16.908–17.982, 15.487–15.721, 37.276–38.603, respectively. Most of the samples plot into the area near EM-Ⅰ, indicating that the magma of the mafic rocks might have derived from a relatively enriched mantle with some crustal materials involved. The geochemical element characteristics of most samples showed typical OIB-type geochemical characteristics indicating that the source region had received metasomatism of recycled materials. Combining with the regional geological background and geochemical data, we inferred that the mafic rocks of the Sugetbrak Formation in the Aksu area were formed in an intraplate rift environment. Summarily, based on our study, the mafic rocks of the Sugetbrak Formation in the Aksu area were derived from a common enriched mantle source, and they were product of a magmatic event during the rift development period caused by the breakup of the Rodinia supercontinent.©2023 China Geology Editorial Office.     

Petrology,geochemistry and a probable late Cambrian age for harzburgites of the Coolac Serpentinite,New South Wales,Australia

The Coolac Serpentinite, in the Tumut region of southeastern NSW, is one of many Alpine-type, linear ultramafic bodies exposed in the Lachlan Orogen of New South Wales. Despite the significance of such oceanic lithosphere throughout the orogen to tectonic models, few studies on the genesis of these bodies in the Lachlan Orogen have been documented. A significant proportion of the Coolac ultramafic rocks are only partially serpentinised, making them good candidates for detailed petrological and geochemical studies. The Coolac peridotites include harzburgites with mineral compositions and bulk-rock REE concentrations similar to abyssal peridotites. Assuming depleted mantle compositions, HREE concentrations are limited (0.2–0.3 × primitive mantle) implying melt extraction of 15–20%. Conversely, some Cr-spinel data within the harzburgites (Cr# = 0.22–0.27) indicate partial melting of only 9–11%. Adsorbed mantle pyroxenes, excess olivine and LREE enrichment suggest melt–rock interactions led to the refertilisation of the harzburgites. Isotope characteristics of a ca 501 Ma allochthonous tonalite block derived from melting of altered oceanic crust and a ca 439 Ma oceanic granite intrusion indicate an identical source that separated from the fertile mantle at 660 Ma. This places chronological constraints on the harzburgites, which are the result of two-stage melting involving a lherzolite protolith formed during the break-up of Rodinia followed by harzburgite formation during a further melt extraction event within an extensional phase of the Delamerian Orogeny. The harzburgites were enriched via melt–rock interactions soon after formation as well as during phases of the Benambran Orogeny beginning at ca 439 Ma and ending around ca 427 Ma with the emplacement of the North Mooney Complex, a layered ultramafic–gabbro association that has characteristics of Alaskan-style intrusions similar to the Fifield complexes of the central Lachlan Orogen.     

天水地区李子园高压麻粒岩的发现:西秦岭晚泥盆纪造山的证据

西秦岭造山带北缘的天水李子园地区发现一套高压石榴石单辉麻粒岩,在变质的早古生代岛弧火山-沉积岩系中呈透镜状岩块产出。其峰期矿物组合为石榴石+单斜辉石+角闪石+斜长石+钛铁矿/榍石。利用地质温压计得到的温压条件为T=757~792℃、P=1.3~1.5GPa,达到中压相系的高压麻粒岩相变质条件。LA-ICPMS锆石U-Pb定年显示,锆石变质增生边的谐和年龄为384±1.6Ma,表明高压麻粒岩相变质作用的时代为中-晚泥盆世。结合区域地质资料,这一期变质作用要晚于北秦岭造山带中普遍记录的中低压麻粒岩-角闪岩相变质作用,可能与商丹洋闭合之后碰撞造山阶段的地壳加厚过程有关。岩浆锆石核部的谐和年龄为796±2.2Ma,代表石榴石单辉麻粒岩的原岩形成年代。其锆石核部的Hf同位素组成变化较大,对应的εHf(t)值为-7.3~+13.2,显示出不同源区岩浆混合的特征或者陆壳混染。高压麻粒岩全岩地球化学特征同样显示其经历壳幔岩浆混合作用。结合原岩的形成时代、区域上与裂解相关的岩浆作用和构造背景,我们认为原岩可能是造山带垮塌伸展阶段的壳-幔岩浆混合作用的产物,可能与新元古代Rodinia超大陆裂解事件有关。