Depositional age and correlation of the Oonah Formation: refining the timing of Neoproterozoic basin formation in Tasmania

The Proterozoic Oonah Formation comprises a thick sequence of turbidites and mafic rocks that is widely exposed in western and northern Tasmania. The regional significance of the Oonah Formation is poorly understood as its contacts are faulted and estimates of its depositional age range from 1450 Ma to 700 Ma. Here we refine the depositional age of the Oonah Formation and clarify its relationship to other Proterozoic sequences in Tasmania. Magmatic apatite from the syn-sedimentary Cooee Dolerite has a ²⁰⁷Pb-corrected ²³⁸U/²⁰⁶Pb age of 733 ± 9 Ma that provides a robust estimate of the depositional age of the Oonah Formation. The Oonah Formation contains abundant 750 Ma detrital monazite and has detrital zircon age populations at 1800–1700 Ma, 1590 Ma, and 1450 Ma. The new age and provenance data support the interpretation that the Oonah Formation is a lateral equivalent of the base of the Togari Group and correlatives in Tasmania. We demonstrate that the stratigraphy and provenance of Neoproterozoic strata in Tasmania differ from time-equivalent sequences in Adelaide Rift Complex but are similar to the Pahrump Group (Death Valley, California) and the Cobham Formation (Transantarctic Mountains), which supports a southwest Laurentia—Tasmania—East Antarctica connection within supercontinent Rodinia.     

Dating of Neoproterozoic and Cambrian orogenies in Tasmania*

A coherent set of timing constraints is produced for Tasmania's Proterozoic and Cambrian geology when only mineral ages are considered and whole‐rock ages excluded. The oldest recognised event is the formation of sedimentary deposits which contain detrital zircons that indirectly indicate a depositional age younger than 1180 Ma. Partial melts of these sedimentary rocks were incorporated in Neoproterozoic, Devonian and probably Cambrian felsic magmas. Neoproterozoic granite on King Island has an age of 760 ± 12 Ma and is part of a high‐level intrusive episode that accompanied the Wickham Orogeny, an event with regionally varied strain that is represented in northwestern Tasmania by a low‐angle unconformity, by altered granitoid with a magmatic age of 777 ± 7 Ma, and by the thick turbidite pile of the Burnie and Oonah Formations with its syndepositional intrusions of Cooee Dolerite. The late Neoproterozoic was relatively quiet tectonically but by early in the Middle Cambrian a crustal collision which marked the early phase of the Tyennan Orogeny brought about high‐level emplacement of ultramafic‐bearing allochthons and deep‐seated metamorphism of quartzose sedimentary and basaltic rocks. The ultramafic allochthons carried tonalite that had crystallised only shortly before at 510 ± 6 Ma, while the deep‐seated metamorphism produced eclogite at 502 ± 8 Ma. By middle Middle Cambrian times the metamorphic rocks had been uplifted and they experienced repeated uplift during the period of Mt Read volcanism and onward to the close of the Tyennan Orogeny in the Early Ordovician, an overall period of some 20 million years from the early Middle Cambrian. Regionally varied strain was again a feature during the Tyennan Orogeny, with the Smithton area in northwestern Tasmania and King Island occupying relatively undeformed cratonic positions.     

河西走廊东部香山群时代和物源讨论

香山群是一套经历轻微区域变质作用的陆源碎屑岩,夹有少量碳酸盐岩和硅质岩,主要位于河西走廊东部香山地区。迄今为止,前人已经做过大量工作,但是对香山群与该区奥陶纪米钵山组的关系、香山群的时代以及香山群的物源等问题还存在较大争议。笔者对香山群的碎屑锆石做了定年(LA-ICP-MS)工作,通过香山群与米钵山组锆石年龄的对比,发现米钵山组最年轻锆石(451±8)Ma比香山群最年轻锆石(525±3)Ma年轻,且香山群不含米钵山组其他年轻锆石成分,香山群时代应该晚于(525±3)Ma,而早于中奥陶世米钵山组,应属中—晚寒武世。香山群碎屑锆石年龄谱与华夏地块、阿拉善基底、华北西部和北祁连岩浆变质事件年龄差异显著,华夏地块、阿拉善基底、华北西部和北祁连不是香山群物源,而与澳大利亚西南部地区相似,同时根据香山群古水流分析结果,推测澳大利亚西南部可能是香山群的沉积物源。     

伊犁盆地南缘中-下侏罗统物源分析及其对南天山造山带演化的启示

伊犁盆地南缘中-下侏罗统碎屑岩的物源特征,可为南天山造山带的演化提供重要证据。对其碎屑岩锆石U-Pb定年研究结果表明,伊犁盆地南缘坎乡下侏罗统八道湾组砂岩的碎屑锆石年龄集中在290~260 Ma,而下侏罗统三工河组的碎屑锆石年龄集中在350~290 Ma和460~390 Ma,中侏罗统西山窑组的碎屑锆石年龄集中在370~320 Ma和450~390 Ma。所有测试样品中前寒武纪的年龄记录非常少。这些特征表明,伊犁盆地南缘中生代碎屑沉积物主要来自于伊犁-中天山地块南部。测试样品中几乎不存在晚二叠世-中三叠世的碎屑锆石,与南天山造山带的岩浆岩记录一致,暗示在晚二叠世-中三叠世南天山地区并没有发生强烈的与碰撞或后碰撞相关的岩浆活动。该结果不支持塔里木克拉通与伊犁-中天山地块在晚二叠世-中三叠世碰撞的观点。结合高压-超高压变质岩的数据和地层记录,认为塔里木克拉通与伊犁-中天山地块的碰撞发生在晚石炭世。同时,样品中最年轻锆石的年龄数据从早侏罗世到中侏罗世逐渐增大,显示了揭顶沉积的特点。对伊犁盆地南部中生代的锆石年龄数据与同时代南天山地区的锆石年龄数据进行综合对比表明在早-中侏罗世发生构造沉积夷平的特征。     

四川米易垭口地区前震旦纪五马箐组与其侵入岩体的锆石U-Pb年代学、Lu-Hf同位素特征及其地质意义SCIEI北大核心CSCD

在扬子板块西缘断续分布有多套变质沉积岩系,厘定区内不同岩石单元的时代对于探讨扬子西缘的构造演化具有重要意义。本文对出露于扬子板块西缘米易垭口地区的五马箐组变沉积岩以及侵入其中的片麻状黑云二长花岗岩体进行了锆石LA-ICP-MS U-Pb定年和Lu-Hf同位素分析。结果表明:区内五马箐组的沉积时代介于1.19~1.01Ga,其地层归属为会理群而非康定群,位于五马箐组之下的“冷竹关组”并非变沉积地层而是一套片麻状黑云二长花岗岩体。五马箐组碎屑锆石年龄存在~1.56Ga、~2.50Ga两个主要峰值以及~1.43Ga、~1.68Ga、~1.87Ga、~2.32Ga、~2.68Ga五个次要峰值,推断其物质来源主要为扬子板块西南缘的撮科杂岩、隐伏的或还暂未发现的太古代基底岩石、同时期岩浆岩和早期变沉积岩的再循环。碎屑锆石的ε_(Hf)(t)值在~1.7Ga发生了显著变化,暗示扬子板块西缘的构造体制由陆壳汇聚向裂谷活动转变,且在~1.56Ga处于裂谷岩浆活动的峰期。花岗岩体正的ε_(Hf)(t)值反映出扬子板块西缘在~1.0Ga有一次新生陆壳生长事件,可能是格林威尔造山后的伸展塌陷构造背景下裂谷岩浆活动的产物。     

Sedimentation, fracturing and sliding on a pelagic plateau margin: the Middle Jurassic to Lower Cretaceous succession of Rocca Busambra (Western Sicily, Italy)

The Jurassic succession of Rocca Busambra consists of two lithostratigraphic units: a pile of peritidal limestones several hundreds of metres thick (Inici Formation: Hettangian to Sinemurian) and a 2 to 15 m thick sequence of Rosso Ammonitico‐type pelagic limestones (Toarcian? to lowermost Berriasian). An extensive interval of non‐deposition is evidenced by a thick Fe–Mn oxide crust on the bounding disconformity and is recorded partially in the material contained within a complex network of neptunian dykes and sills. Seven lithofacies are distinguished in the Rosso Ammonitico. These lithofacies show that the Rosso Ammonitico limestones differ from most analogues both in Sicily and elsewhere: sediments are mostly grain‐supported and non‐nodular; obviously bottom currents were important during deposition of these sediments. These currents were pulsating at different frequencies and induced winnowing, intraclast production and early cement precipitation. Other Rosso Ammonitico lithosomes of Late Jurassic to earliest Cretaceous age, usually decimetre thick and discontinuous, overlie the Inici Formation without any Fe–Mn crust; their anomalous stratigraphical and geometrical relationships show that they were deposited on an inclined, stepped, erosional surface incised in the sub‐horizontal Inici Formation. This ancient escarpment is interpreted as the result of a mainly gravitational collapse of the margin of a pelagic plateau. Such mass wasting was probably due to the backstepping of the tectonic plateau–basin margin that is not observable directly, but may be inferred from circumstantial evidence. This observation clearly shows that tectonic activity affected the Rocca Busambra sector of the West Tethys continental margins a few tens of millions of years after the end of the rifting stage. The anomalous Rosso Ammonitico sediments are the only indication of the escarpment and their occurrence in the stratigraphic record is probably more widespread than reported in the literature. More accurate palaeoenvironmental and palaeogeographic reconstructions may depend on the identification of these sediments.     

Dolerite intrusion morphology at Majuba Colliery, northeast Karoo Basin, Republic of South Africa

Dolerite sills, at times transgressive, and dykes are common at Majuba Colliery. Their behaviour within the Karoo stratigraphic pile limits and controls the effectiveness of extracting the deep-seated Gus coal seam. Due to the intrusion of dolerites, the coal seam elevation can vary by as much as 70 m. Data from 452 boreholes, 88 of which were drilled to granitic gneiss basement and data from the underground development were used to construct cross-sections through the colliery. Based on texture, geochemistry and mode of emplacement, there exist four different dolerite types (T1 to T4) at Majuba. These are intruded into sedimentary rocks of the Karoo Supergroup although one, the T3 dolerite, has been found intruded into basement gneiss. In the east, an ultramafic intrusion of pre-Karoo Supergroup age created a basement high. By far, the greatest number of dolerite sills intruded within a sandstone unit, and have identical sandstones in the footwall and hangingwall, rather than intruding along lithological boundaries.     

川滇藏交界区二叠纪—早三叠世的两套弧火山岩

川滇藏交界区属三江印支造山带中段,地质构造十分复杂.文章据最近获得的岩石化学及野外调查资料分析了该区古特提斯活动大陆边缘的两套弧火山岩的特征,结果表明:早二叠世的吉东龙组是碧土洋盆向东俯冲的记录,当时的活动大陆边缘属西太平洋岛弧型;晚二叠-早三叠世的夏牙村组-马拉松多组是金沙江洋盆(南段)向西消减的证据,当时的活动大陆边缘接近安第斯型.在此基础上讨论它们在恢复古特提斯演化的时空制约中的意义.     

北祁连东段景泰地区下古生界两套砂岩微量元素和稀土元素特征及其构造意义

早奥陶世和早志留世是北祁连加里东造山带构造演化和盆地转变的关键时期。在造山带东段景泰地区,下奥陶统阴沟组和下志留统肮脏沟组两套砂岩的微量元素和稀土元素特征显示,阴沟组杂砂岩样品（Cj1和Cj3）具有最小的Eu/Eu*及最大的Th/Sc和REE,肮脏沟组杂砂岩具有较小的Eu/Eu*和较大的Th/Sc及REE;阴沟组岩屑砂岩样品（Cj13、Cj15和Cj18）具有最大的Eu/Eu*及最小的Th/Sc、REE和La/Yb。多个物源、构造背景判别图解和多元素蛛网图分析表明,阴沟组杂砂岩样品具大陆边缘的构造背景,主要物源为大陆上地壳再旋回沉积物和长英质岩石;岩屑砂岩样品为岛弧构造背景,以中基性安山质岩石为主要物源,可能受陆源物质的微弱影响。肮脏沟组杂砂岩构造背景复杂,表现出大陆岛弧、活动陆缘和被动陆缘三种环境共存的特点,受中基性火山弧物质、长英质岩石和再旋回沉积岩的混合物源的影响。两套砂岩的元素特征表明二者可能具有相似的源区。阴沟组杂砂岩源区可能为阿拉善地块南缘海原群变沉积岩或其他相似的陆源再旋回沉积物,砂岩碎屑以来自初始火山弧物质为主,以石灰沟岛弧型中基性火山岩作为其源岩最合适。阴沟组形成于初始弧后盆地环境,是岛弧活动的直接记录。肮脏沟组可能的源岩为阿拉善地块南缘海原群变沉积岩和中高等成熟度的石灰沟岛弧型火山岩及海原群岛弧型变火山岩,沉积于弧后前陆盆地,对构造环境的反映存在滞后性。     

U–Pb ages and Hf isotopic record of zircons from the late Neoproterozoic and Silurian–Devonian sedimentary rocks of the western Yangtze Block: Implications for its tectonic evolution and continental affinity

The relationship of the Yangtze Block with other continental blocks of the Rodinia and Gondwana supercontinents is hotly debated. Here we report U–Pb and Lu–Hf isotopic data for zircons from the latest Neoproterozoic Yanjing Group and the overlying Silurian–Devonian rocks on the western margin of Yangtze Block, which provide critical constraints on the provenance of these sediments and further shed light on the crustal evolution and tectonic affinity of the western Yangtze Block in the context of Rodinia and the subsequent Gondwanaland. Mica schist from the middle part of the Yanjing Group contains dominant Neoproterozoic detrital zircons (0.72–0.80 Ga) with a pronounced age peak at 0.75 Ga. Based on the euhedral to subhedral shapes, high Th/U ratios and exclusively positive εHf(t) values (+ 6 to + 14) for the zircon crystals, and the lack of ancient zircons, we consider the sediments as products of proximal deposition near a Neoproterozoic subduction system in western Yangtze. Combined with the age of rhyolite from the lower part of the Yanjing Group, these strata were estimated to have been deposited in a period between 0.72 and 0.63 Ga. In contrast, the Silurian–Devonian sediments exhibit dominant Grenvillian ages (0.9–1.0 Ga), with middle Neoproterozoic (0.73–0.85 Ga), Pan-African (0.49–0.67 Ga) and Neoarchean (~ 2.5 Ga) age populations, suggesting a significant change of sedimentary provenance and thus a different tectonic setting. Although the shift occurred in the Silurian, the age spectra turn to be consistent along the western margin of the Yangtze Block until the Devonian, indicating persistence of the same sedimentary environment. However, the related provenance of these Paleozoic sediments cannot be found in South China. The presence of abundant Grenvillian, Pan-African and Neoarchean ages, along with their moderately to highly rounded shapes, indicates the possibility of exotic continental terrane(s) as a possible sedimentary provenance. Considering the potential source areas around the Yangtze Block when it was part of the Rodinia or Gondwana, we suggest that the source of these Paleozoic sediments had typical Gondwana affinities such as the Himalaya region, north India, which is also supported by their stratigraphic similarity, newly published paleomagnetic data and the tectono-thermal events of northwestern fragments of Gondwana. This implies that after a prolonged subduction in the Neoproterozoic, the western margin of the Yangtze Block began to incorporate into the assembly of the Gondwana supercontinent and was able to accept sediments from northwestern margin of Gondwanaland as a result of early Paleozoic orogeny.     

华北地块东北缘红庙子盆地鹰嘴砬子组孢粉化石

报道华北地块东北缘地区红庙子盆地鹰嘴砬子组的孢粉化石,建立了Cicatricosisporites-Abietineaepollenites-Piceites组合,该组合的主要特征为:以裸子类双气囊花粉和海金沙科孢子为主,含一定量裸子类无气囊花粉,未见被子类花粉,孢粉组合特征可与松辽盆地沙河子组对比。一些具有早白垩世时代意义的孢粉化石,如Cicatricosisporites sp.,Cicatricosisporites undulates和Cicatricosisporites implexus的出现,表明鹰嘴砬子组的地质时代为早白垩世。根据孢粉化石资料,推测华北地块东北缘地区在鹰嘴砬子组沉积时期古植被面貌以松科和杉科组成的松柏类针叶林为主,林下生长着真蕨类。古气候相当于湿润-半湿润的亚热带气候。     

El cretácico medio y superior de la parte meridional del Bloque Maya (Guatemala)

Study of the Cretaceous strata of the Altos Cuchumatanes (western Guatemala) and in Alta Verapáz (central Guatemala) has revealed a number of foraminifers never before reported from these areas. These data permit a similar nomenclature to be applied to both areas in regard to the division of the sequence into formations. Thus, we distinguish in both areas: a) the Cobán Formation, a shallow platform carbonate of early Aptian to early Senonian age; b) the Campur Formation, of Campanian age, composed of a lower carbonate member with rudists and large benthonic foraminifers, and an upper carbonate member with pelagic foraminifers; c) the Sepur Formation, of Maastrichtian age, with flysch-like sediments, including intercalated blocks of ophiolite complex obducted during the Maastrichtian. We conclude that in early Aptian time, a shallow carbonate platform developed along the southern margin of the Maya Block. The platform subsided and was drowned during the terminal Campanian. This subsidence led to the formation of a flysch-like basin that received turbiditic sedimentation from the carbonate platform to the north and fragments of serpentinite from the ophiolite complex to the south.     

Precambrian crustal contribution to the Variscan accretionary prism of the Kaczawa Mountains (Sudetes,SW Poland): evidence from SHRIMP dating of detrital zircons

SHRIMP dating of detrital zircons from sandstones of the Gackowa Formation (Kaczawa Complex, Sudetes, SW Poland) indicates input from late (550–750 Ma) and early Proterozoic to Archaean sources (∼2.0–3.4 Ga, the latter being the oldest recorded age from the Sudetic region). These dates preclude within-terrane derivation from seemingly correlatory acid volcanic rocks of early Palaeozoic age. Rather, they indicate provenance from Cadomian and older rocks that currently form part of other, geographically distant terranes; the most likely source identified to date is the Lusatian Block in the Saxothuringian Zone. Hence, the Gackowa Formation may be late Proterozoic rather than early Palaeozoic in depositional age, possibly coeval with the late Proterozoic (pre-Cadomian) greywackes of Lusatia, being subsequently tectonically interleaved with early Palaeozoic volcanic rocks into the Kaczawa accretionary prism during the Variscan orogeny. However, correlation with the lithologically similar early Ordovician Dubrau Quartzite of Saxothuringia, and so assignation to the early Paleozoic (post-Cadomian) rift succession deposited at the northern margin of Gondwana, cannot yet be precluded.     

Lower Paleozoic detrital zircon U-Pb geochronological characteristics of the Baoshan Block in western Yunnan and its constraints on the Proto-Tethys tectonic evolutionSCIEI北大核心CSCD

滇西保山地块是东特提斯构造域的主要微陆块之一,但对其物源和古地理位置仍存在较大争议。本文通过对保山地块西缘早古生代地层进行碎屑锆石U-Pb定年来约束其物源及古地理位置,并进一步探讨原特提斯洋早古生代构造演化模式。保山地块西缘早古生代地层具有相似的年龄分布模式,主年龄峰期为-0.95Ga、次级年龄峰期为-1.2Ga和-2.5Ga。寒武系公养河群最小锆石年龄为526Ma,结合其上部年龄为499.2Ma的火山岩夹层,约束其沉积时代为早寒武世早期。对比保山地块不同区域早古生代地层的碎屑锆石年龄数据,它们都具有相似的锆石年龄分布模式和年龄峰值。-0.95Ga主年龄峰期和-2.5Ga的次级年龄峰期指示保山地块早古生代的沉积物主要来自于印度大陆,而-1.2Ga的次级年龄峰期表明有部分沉积物来自于西澳大利亚,其早古生代古地理位置位于印度和西澳大利亚之间。结合沉积学证据及滇西地区广泛发育的早古生代岩浆作用,本文认为早古生代冈瓦纳大陆北缘为活动大陆边缘。     

The age and composition of the pre-Cenozoic basement of the Jalisco Block: implications for and relation to the Guerrero composite terrane

The Jalisco Block is thought to be part of the Guerrero terrane, but the nature and age of the underlying crystalline basement are largely unknown. We have collected a suite of schists, granitoids, and weakly metamorphosed marine sediments from various parts of the Jalisco Block including Atenguillo and Ameca, Mascota and San Sebastián, Cuale, Puerto Vallarta, Punta Mita, Yelapa, and Tomatlán. The schists range in age from 135 to 161 Ma, with many exhibiting Proterozoic and Phanerozoic zircon ages. The granitoids range in age from 65 to 90 Ma, and are calc-alkaline compositionally—similar to granitoids from the Puerto Vallarta and Los Cabos batholiths. The Jalisco granitoids also experienced similar uplift rates to granitoids from the regions to the north and south of the Jalisco Block. The marine sediments yield a maximum depositional age of 131 Ma, and also contain a significant zircon population with ages extending back to the Archean. Granitoids from this study define two age groups, even after the effects of thermal resetting and different closure temperatures are considered. The 66.8-Ma silicic ash flow tuff near Union de Tula significantly expands the extent of this Cretaceous–Paleocene age ash flow tuff unit within the Jalisco Block, and we propose calling the unit “Carmichael silicic ash flow tuff volcanic succession” in honor of Ian Carmichael. The ages of the basement schists in the Jalisco Block fully overlap with the ages of terranes of continental Mexico, and other parts of the Guerrero terrane in the south, confirming the autochthonous origin of the Jalisco Block rather than exotic arc or allochthonous origin. Geologic data, in combination with geochronologic and oxygen isotopic data, suggest the evolution of SW Mexico with an early 200–1,200-Ma passive margin, followed by steep subduction in a continental arc setting at 160–165 Ma, then shallower subduction by 135 Ma, and finally, emplacement of granitoids at 65–90 Ma.     

浙江江山寒武系碎屑锆石U-Pb年龄与沉积指向构造及其古地理意义

浙江江山北部处于紧邻江山-绍兴断裂带的扬子地块边缘;寒武系杨柳岗组陆源碎屑岩夹层中的碎屑锆石 LA-ICP-MS U-Pb定年结果表明,年龄范围为515~3340 Ma,主峰年龄828 Ma,其次为727 Ma及916 Ma;与邻区的对比结果 显示,研究区碎屑锆石年谱与华夏地块闽西北-赣中东部以及扬子地块赣东北-皖南地区可对比,尤其与前者最为相近, 而与华夏地块闽西南-赣南地区差异显著。杨柳岗组及其上覆华严寺组同沉积褶皱以及下伏大陈岭组单向斜层理特征表 明,沉积基底向北西倾斜,沉积物搬运方向由南东向北西。综合这两方面推测：在杨柳岗组沉积时期,研究区具有向北西 倾斜的沉积基底,向东南方向应与剥蚀区相连,陆源碎屑物应源自其东南方向的华夏地块东部;剥蚀区可能位于现今包括 江绍构造带在内的浙江中部,向北西方向主要提供前寒武系陈蔡群和相当于河上镇群以及少量双溪坞群或者三者剥蚀物再 沉积地层的剥蚀物质。     

湘西花垣排碧寒武系花桥组上段—车夫组沉积环境的探讨

湘西排碧位于中上扬子陆块的东南缘,前人认为在寒武纪第三世时期该地区位于以各种类型的"碳酸盐重力流沉积"发育为显著特征的"台缘斜坡相带"内,其主要岩石类型为泥质条带灰岩及砾屑灰岩。本文通过对花桥组上段—车夫组典型沉积物的岩石组分及沉积构造进行详细研究,发现条带灰岩并非泥质条带灰岩,而是由灰岩条带——颗粒灰岩、泥晶灰岩与粉砂质条带——粉砂质灰岩、纹层状含炭质粉砂质灰岩互层组成,表现出内源碳酸盐沉积物与陆源碎屑沉积物混积以及陆棚环境沉积物的典型特征。砾屑灰岩及伴生岩石组合发育丘状(洼状)交错层理等众多风暴成因的沉积构造,为具有不同风暴沉积序列的风暴沉积物。此外,花桥组上段—车夫组沉积物中含各种藻类等浅水生物及大量三叶虫骨刺。因此,认为花桥组上段—车夫组的沉积环境应为正常浪基面以下、风暴浪基面以上,受周期性强风浪作用影响的混积陆棚环境。     

华南扬子陆块西缘新元古代康滇裂谷盆地开启时间新证据

苏雄组为华南扬子陆块西缘新元古代康滇裂谷盆地开启初期的火山喷发产物,其形成时限可为制约裂谷盆地开启时间提供新证据。本文对川西汉源九襄地区新元古代苏雄组流纹岩进行了LA-ICP-MS锆石U-Pb同位素测定。结果显示,测试样品锆石U-Pb谐和年龄为853 ±2 Ma（N=13;MSWD=3.1）,加权平均年龄为853 ±4 Ma（N=13;MSWD=2.2）。新的年龄数据表明,苏雄组的底界年龄应该下延至ca. 850 Ma,扬子西缘新元古代裂谷盆地开启时间应更改为ca. 850 Ma,与全球超级地幔柱上涌和Rodinia超大陆裂解时间基本同步。     

四川会理地区古近纪雷打树组碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

采用LA-ICP-MS方法,对四川盆地南部会理地区古近纪雷打树组碎屑锆石进行了U-Th-Pb同位素测定,获得了72组单颗粒锆石的U-Pb年龄,建立了碎屑锆石的U-Pb年龄谱。结果表明,雷打树组碎屑锆石U-Pb年龄区间为2465~204Ma,地质时代为古元古代最早期成铁纪至晚三叠世最晚期瑞替阶,年龄分布具有清晰的幕式分布特征,集中分布于5个区间,出现了5个明显的峰值,物源区主要为扬子陆块西缘及其西侧的“三江”造山带。雷打树组碎屑锆石U-Pb年龄谱显示,扬子陆块西缘经历了古元古代陆壳增生、中元古代Rodinia超大陆汇聚、新元古代晚期Rodinia超大陆裂解、二叠纪玄武岩喷溢及中-晚三叠世印支运动5次重要的构造热事件,与扬子陆块西缘形成演化进程完全吻合。与四川盆地古近纪柳嘉组碎屑锆石的U-Pb年龄谱相比,雷打树组碎屑锆石U-Pb年龄谱缺失侏罗纪、白垩纪信号,增加了早奥陶世和早泥盆世信号,说明四川盆地北部与南部的物源存在一定的区别。碎屑锆石U-Pb年龄谱对比结果显示,雷打树组碎屑锆石U-Pb年龄谱具有较高的精确度,扬子陆块与华夏陆块自1000Ma汇聚以来具有很好的亲缘性,而与华北克拉通之间直至400Ma才开始建立亲缘关系。     

Glacigenic Characteristics of the Luoquan Formation and Sediment Gravity Flow Reworking on It

The origin of the Luoquan Formation which occurs along the southern margin of the North China Blockhas long been argued. Based on recent work. the Formation is considered as a glacial sedimentary sequencepartially reworked by sediment gravity flow. The major evidence for the glacigene of Luoquan Formationdiamictites is as follows: 1, a striated and polished pavement with various features resulting from glacialabrasion and plucking, such as crescentic gouge, crescentic fracture, streamlined form and glaciated step; 2.unsorted diamictites with striated clast. faceted clast and iron-shaped stone formed by glaciation; 3. rhythmitewith dropstones; 4. a glacial sedimentary sequence bearing advance-retreat cycles; and 5. wide distribution ofthe diamictites. Glacial deposits can be distinguished from sediment gravity flow deposits by the features men-tioned above. Some characteristics of sediment gravity flow existing in the Luoquan Formation diamictites in-dicate that glacial deposits might have been partially reworked by sediment gravity flow. Therefore, this papersuggests that the Luoquan Formation diamictite is a result of a glacial event rather than a mud flow deposit.The primary tillites are the principal contribution of the Luoquan Formation, while sediment gravity flow de-posits are the redeposited diamictites and should be termed as glacigenic sediment gravity flow deposits.