Petrogenesis of Late Silurian volcanism in SW Yunnan (China) and implications for the tectonic reconstruction of northern Gondwana

ABSTRACT

Early Palaeozoic magmatic records and tectonic reconstructions along the northern margin of Gondwana are still pending problems. In this paper, Late Silurian Dawazi and Dazhonghe volcanics in SW Yunnan Province (China) were studied. The Dazhonghe volcanics (419 Ma) have variable chemical compositions with SiO₂ ranging between 49.8 and 79.5 wt.%, whereas, the Dawazi volcanics (417–429 Ma) form a bimodal volcanic suite consisting mainly of silicic rocks and subordinate basaltic rocks with a SiO₂ content gap of ca. 15 wt.%. The Dazhonghe volcanics display calc-alkaline elemental compositions with enrichment in light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs) (e.g. Nb, Ta and Ti) and positive ε_Nd(t) values (+ 4.0 to + 5.5). The Dawazi basaltic rocks are calc-alkaline, depleted in HFSEs, enriched in large ion lithophile elements (LILEs) (e.g. Cs, Rb, U and K), and have high ε_Nd(t) values of ?1.7 to + 5.4. The Dawazi silicic rocks have high Na₂O/K₂O ratios and positive ε_Nd(t) values of + 2.4 to + 5.0, which are equivalents of calc-alkaline I-type granites. The Dazhonghe volcanics are dominated by fractional crystallization (FC) from a calc-alkaline primary magma which originated from an enriched mantle source metasomatized by subduction-related, sediment-derived fluid. The Dawazi basaltic rocks were derived from partial melting of an enriched mantle source metasomatized by subducted oceanic sediment/slab-derived fluids; the Dawazi silicic rocks originated from partial melting of the juvenile mafic lower crust with extensive FC. Both the Dazhonghe and Dawazi volcanics were generated in a continental back-arc extension setting. Combined with previous geological observations, a Late Silurian Prototethyan arc and back-arc extension system is proposed along the northern margin of Gondwana in SW China.     

再论华南志留系红层的时代

牙形刺P.eopennatus带的确立和P.celloni带、P.amorphognathoides带的细分,使华南Llandovery统的划分和对比发生了变化,以往归入P.celloni带的地层,极大多数都要归属到P.eopennatus带,层位变低。依据牙形刺生物地层的分析,溶溪组（下红层）的时代可能为埃隆期晚期,...     

Late Silurian to Early Devonian Palynomorphs from Qujing, Yunnan, Southwest China

Well-preserved and diversified spores, cryptospores, and acritarchs have been recorded from a relatively continuous sequence that encompasses the Silurian–Devonian boundary in Qujing, Yunnan, southwest China. Four spore assemblage zones from Late Silurian to Early Devonian in age are proposed based on the first appearance datum (FAD) of characteristic spore species. In ascending stratigraphic order, they are Ambitisporites dilutus–Apiculiretusispora synorea (DS; Late Ludfordian to Early Pridoli), Synorisporites verrucatus–Apiculiretusispora plicata (VP; Pridoli), Apiculiretusispora minuta–Leiotriletes ornatus (MO; Lochkovian), and Verrucosisporites polygonalis–Dibolisporites wetteldorfensis (PW; Pragian). The acritarch assemblage from the upper part of the Yulongsi Formation, the Xiaxishancun Formation, and the lower–middle parts of the Xitun Formation indicates an age of Late Silurian. Based on palynological evidence, the upper part of the Yulongsi Formation is considered Late Ludfordian to Early Pridoli in age; the Xiaxishancun Formation is believed to be Pridoli in age; the Xitun Formation is considered Late Pridoli to Early Lochkovian in age; the Guijiatun Formation is considered Lochkovian in age; and the Xujiachong Formation is Late Lochkovian to Pragian in age. The Silurian-Devonian boundary is recognized between the VP and the MO spore biozones, and occurs within the middle part of the Xitun Formation.     

Evolution of an intra‐oceanic island arc during the Late Silurian to Late Devonian,New England Fold Belt

Geochemical studies of volcanic rocks in the Gamilaroi terrane and Calliope Volcanic Assemblage, New England Fold Belt, eastern Australia, indicate that the setting in which these rocks formed changed in both space and time. The Upper Silurian to Middle Devonian basalts of the Gamilaroi terrane show flat to slightly light rare‐earth element (LREE) depleted chondrite normalised patterns, depletion of high field strength elements (HFSE) relative to N‐MORB, low Ti/V and high Ti/Zr ratios, high Ni, Cr and large‐ion lithophile element (LILE) contents, features characteristic of intra‐oceanic island arc basaltic magmas. They are associated with low‐K, less mafic volcanics, showing moderate LREE enrichment, low Nb and Y contents and Rb/Zr ratios. The depletion of HFSE in the basalts indicates that the magmas were derived from a refractory source in a supra‐subduction zone setting. The presence of such a zone implies that the arc was associated with a backarc basin, the location of which was to the west where a wide backarc region existed from the Middle Silurian. This polarity of arc and backarc basin suggests that the subduction zone dipped to the west. In contrast to their older counterparts, Middle to Upper Devonian basalts of the Gamilaroi terrane have MORB‐like chondrite normalised patterns and higher Ti and lower LILE contents. Moreover, they have low Ti/Zr ratios and MORB‐like Ti/V ratios and HFSE contents, features typical of backarc basins. Dolerites of the Gamilaroi terrane also have predominantly backarc basin signatures. These features suggest that both the basalts and dolerites have been emplaced in an extensional environment produced during the rifting of the intra‐oceanic island arc lithosphere. A progressive increase in Ti/V ratios, and TiO₂ and Fe₂O₃ contents at constant MgO, of stratigraphically equivalent basalts, towards the north‐northwest part of the belt, is consistent with either greater extension to the north or melting of a more fertile magma source. By contrast, basalts in the southeast part of the terrane have moderately high Ti/Zr and low Ti/V ratios and in some samples, exhibit depletion of HFSE, compositional features transitional between island arc and backarc basin basalts. The Lower to Middle Devonian mafic rocks in the Calliope Volcanic Assemblage show both LREE enriched and depleted chondrite normalised REE patterns. Further, the majority have high Ti/Zr ratios and low Zr contents as well as relatively high Th contents relative to MORB. These features are common to rocks of Middle Devonian age as well as those of Early Devonian age and are suggestive of eruption in an arc setting. Thus, the data from this study provide new evidence for the evolution of the New England Fold Belt from the Late Silurian to the Late Devonian and reveal a history more complicated than previously reported.     

Chronology of landscape evolution and soil development in the upper Flinders River area,Queensland, based on isotopic dating of Cainozoic basalts

Victorian granites containing more than 750 ppm Ba are almost entirely confined to the region between a line from Geelong to Swan Hill and the Wonnangatta Fault Zone. Granite Ba contents normalised to 70% SiO₂ range from 620 to 733 ppm in western Victoria, 719 to 1560 ppm in central Victoria and 493 to 689 ppm in eastern Victoria. Melting of Ba-rich (meta)sedimentary rocks in the lower – middle crust is implicated in the petrogenesis of central Victorian S types, at least. Thus, granite geochemistry supports the concept of some sort of (largely concealed) Ba-rich Selwyn Block beneath central Victoria, although the boundaries that have been proposed for the block are modified here. There is a strong possibility that the Selwyn Block is an exotic terrane emplaced by northwest-directed strike-slip movement during the Bindian Orogeny. Such movement appears to have been controlled by the previously postulated Baragwanath Transform and another fundamental fault here called the Ulrich Transform. Asthenospheric upwelling related to movement on the Ulrich and Baragwanath Transforms may be the explanation for the twin belts of 400 Ma plutonism occurring to the west of the former and to the east of the latter. The southern extension of the Ulrich Transform may be the Tamar – Tiers structure in Tasmania. Plate-tectonic models suggesting Ordovician – Silurian subduction in Victoria need to be carefully revisited given the possibility of Siluro-Devonian exotic terrane emplacement.     

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt,southeastern Australia

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison with backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inversion reflects advance of blocks driven by compression related to the Indian collision.     

The Devonian units of the Marimanya massif and their relationship with the Pyrenean Devonian facies areas

Abstract

The metasedimentary materials of the Marimanya Massif are chiefly made up of Devonian deposits, classically attributed to the Cambro-Ordovician. Their stratigraphic succession has been reconstructed from geological mapping, structural analysis and the study of the limestone conodont content, and subsequently divided into four Devonian units.

The lower part consists of the Moredo-Salau carbonatic unit (braided alternation, crinoidal carbonates, parallel alternation and Campaus limestones). Its age ranges from the Upper Silurian to Lower Emsian. This unit is overlain by lutites with limestone and fine sandstone intercalations from the Beret series (Lower Emsian-Lower Frasnian).

The two units are correlated in the east with the Salau carbonate series and the “Shales Bleutes”, and in the Vielha synclinal with the “Calcaire Basal” and the Entecada, Auba, Sa Fusta and Mont Corbisum units.

The Upper part of the succession is formed by the Montgarri and Can Cabau Detrital units, which are lithologically correlated with the Sa Cal and Riu Nere units in the Vall d’Aran and, in the west, with the Agudes-Cap de Pales and Sia series.

Thus the Devonian Central facies area stretches eastwards to the Pyrenees ariégeoises and westwards to the Vielha syncline, Les Agudes, Sia, Sagette and Laruns.

The new biostratigraphic succession established in the Marimanya Massif represents, to date, the most complete within the Central facies area. The new data obtained enable the correlation between the Devonian units in this facies area with the units in the remaining Devonian facies area in the Pyrenees.     

Palaeotopography of the northern portion of the Telychian (Silurian) turbidite basin in central Wales

New palaeocurrent data from the country northeast and southwest of Corris (central Wales) indicate that Telychian sandstones in the Devil's Bridge Formation hereabouts were transported from both the northeast and northwest, consistent with topographical control by down‐to‐southeast movement on the Bala Lineament. This conclusion is supported by considerations of thickness variation and by evidence for listric detachment faulting in the formation at Bwlch y Groes. Towards Plynlimon, flow is broadly southerly and suggests confinement by the northward extension of the Bronnant Fault. A facies and palaeocurrent map is presented for the utilis sub‐Biozone of the Telychian which links with work by the British Geological Survey between Aberyswyth and Rhayader where, by contrast, palaeoflow is to the NNW. Copyright © 2005 John Wiley & Sons, Ltd.     

四川盐边稗子田志留系与泥盆系的界线划分及地质意义

四川省盐边县北部稗子田地区的专留系发育良好,牙形刺化石极为丰富,与上覆泥盆系连续沉积,并呈整合接触。该剖面为我国扬子区较为理想的志留纪地层剖面,其代表和反映了扬子区西部一种新的断陷盆地碳酸盐岩沉积类型,丰富了区内志留系的研究内容。笔者详细记述了稗子田专留系至下泥盆统下部剖面的岩性特征和生物化石的垂直分布状部,在此基础上建立和完善区内新的地层系统,提出地层划分的对比依据和建议。新方案除对志留系底的黑     

塔里木盆地塔中、塔北地区志留系古油藏的油气运移

运用含氮化合物咔唑类探讨塔中、塔北地区在第一期成藏时志留系油气藏的烃类的运移方向, 为建立古油藏成藏模式, 预测原生油气藏分布提供依据.研究表明, 塔中志留系古油藏的油气主要来源于满加尔凹陷的中、下寒武统烃源岩, 向西南—南的方向首先进入塔中志留系, 然后沿不整合面或顺储层从北西向南东和从北东向南西2个方向向志留系在塔中地区的尖灭线附近运移.塔北志留系古油藏的油气也主要来自满加尔凹陷的中、下寒武统烃源岩, 首先向北西方向进入塔北隆起的志留系, 然后在志留系储层内或不整合面沿上倾方向继续向北西方向运移进入圈闭.