Palaeozoic sequences and evolution of the Calabrian-Peloritan Arc (Southern Italy)

Three Palaeozoic sequences belonging to three different basins crop out in the Calabrian–Peloritan Arc. Their age covers the time span from middle (?) Cambrian to early Carboniferous. The sequences comprise terrigenous, volcanic and carbonate rocks, and show low-grade metamorphism. The basement is of pre-middle Cambrian age, crops out in the Calabrian–Peloritan Arc, and was metamorphosed prior to the opening to the Palaeozoic basins. The Palaeozoic basins existed no later than the middle Carboniferous, with inferred maximum crustal extension in the Cambro-Ordovician. By Devonian time, the tectonic regime was compressional overall with middle Devonian island-arc type volcanic activity that continued until the closure of the basins. Approximately 330 Myr, the Palaeozoic sequences experienced low-pressure greenschist facies metamorphism and continuing subduction controlled the Variscan tectonogenesis with 280 Myr island-arc type intrusive magmas. Subduction ceased and late-stage 280–270 Myr granitic magmas were emplaced during continental collision. The crustal sector carrying the Palaeozoic basins is interpreted as comprising fragments of an active continental margin, one of the several microplates, located at the southern margin of the Euro-Asia continent which faced a large (at least 1500 km in width) ocean basin.     

Ordovician volcanism of the Peloritan Mountains (Sicily): Implications for evolution of Palaeozoic basins in the Calabrian-Peloritan Arc

The palaeontologically dated Ordovician associations of the Peloritan Mountains consist of metasilts, metapelites, calc-schists and metavolcanic rocks. These rocks form a portion of a terrigenous-volcanic-carbonate sequence of Cambro-Ordovician to Carboniferous age. This Palaeozoic sequence occupies the lower tectonic position in a Variscan orogen, affected by later Alpine deformation. The upper portion of this orogen is formed by low to high grade metamorphic rocks and some rare magmatic rocks. The Ordovician section of the sequence is divisible into a lower part (probably Arenigian), characterized by frequent and extensive within-plate alkaline metabasalts, and an upper part characterized by metadacites and metarhyolites intercalated with metasediments, commonly carbonates. The chemical characteristics of the metavolcanic rocks and the composition and structure of the metasediments indicate the persistence of tensional conditions during the Lower Ordovician. In contrast, the Upper Ordovician was marked by reducing tension, stagnation of basaltic magmas at different crustal levels, and consequent formation of dacitic and rhyolitic melts by partial melting of the surrounding crust. A comparison of the Ordovician sequences of the Peloritan Mountains with those of the Calabrian Palaeozoic basins shows several geodynamic similarities. Among other circum-Mediterranean basins, only that of north-western Bulgaria shows good analogies.     

The devonian and carboniferous volcanism of the peloritan mountains (sicily) and the evolution of palaeozoic basins in the calabrian-peloritan arc

Volcanic rocks from three Palaeozoic low-grade metamorphic sequences of different age from the Peloritan Mountains, Sicily, have been analysed for major and trace elements. On the basis of the relative abundances of certain comparatively immobile elements (especially Zr, Ti, Nb, and Y) the petrogenetic affinities of the volcanic rocks in each area have been established. The Fiumara Fitalia sequence contains Lower Devonian within-plate alkaline basalts, the Ogliastrello sequence contains Upper Devonian continental within-plate calc-alkaline basalts and the Randazzo-Floresta road sequence contains Lower Carboniferous calc-alkaline basalts probably of island arc affinity. When account is taken of their stratigraphic positions, geochemical characteristics of the volcanic rocks indicate Devonian extensional conditions and Lower Carboniferous compressional conditions. Evidence for Lower Devonian extension is present in several circum-Mediterranean basins (i.e. Calabria, Sardinia, Carnic Alps). In the Peloritan Mountains, as well as in Calabria and Sardinia, extensional conditions were already present in the Cambro-Ordovician. The compression in the Calabrian-Peloritan Arc was probably related to a period of subduction during the Hercynian orogeny and resulted in the closure of the basins in the Carboniferous.     

贺兰山中段早古生代地层旋回层序研究

研究区位于鄂尔多斯西缘凹陷带的贺兰山中段地区,寒武—奥陶系地层出露齐全。作者在综合了野外特征和室内分析的基础上,将该区沉积地层划分为三大沉积体系和十四种沉积相类型,并分述了每一沉积相类型的岩性组合及其古生物化石、沉积构造和沉积相序特征,进而对区内早古生代地层的旋回层序进行了分析,认为从早寒武世开始至早奥陶世结束为一个Ⅱ级旋回层序,其中包括四个Ⅲ级海平面变化周期;从中奥陶世开始至其结束。代表另一个独立的受古构造因素控制的Ⅱ级旋回层序,其中每一内源重力流单元与陆源重力流单元的规则变化则相当于一个Ⅲ级旋回层序。     

Detrital rutile U-Pb perspective on the origin of the great Cambro-Ordovician sandstone of North Gondwana and its linkage to orogeny

LA-ICPMS U-Pb geochronology of detrital rutile from North Gondwana Cambro-Ordovician sandstones is reported for the first time. Our sampling sites in Israel, Jordan and Ethiopia are situated thousands of kilometers apart thus providing a continental-scale perspective on the cooling and unroofing history of the provenance of the great Cambro-Ordovician sandstone sheet.Detrital rutiles in the Cambrian sandstone of Israel and Jordan define a unimodal age concentration at 0.59–0.58 Ga, preceding the timing of major orogenic sutures and predating the final assembly of Gondwana. Cooling to below the rutile U-Pb closure temperature followed continental-scale Pan-African granitoid intrusion at 0.63–0.60 Ga, signifying that rather than representing high-grade metamorphism the detrital rutile age spectra reflect widespread resetting, cooling and exhumation associated with late to post-tectonic igneous activity. The unimodal 0.59–0.58 Ga age spectra do not change significantly up the Cambrian sequence, implying exhumation prior to Cambrian sedimentation generated ample crustal material with an almost uniform U-Pb rutile signature that was prone to deliver immense amounts of detritus. Therefore, rather than reflecting progressive erosion of Cambrian Supermountains, the North Gondwana Cambrian sandstone represents secondary denudation of an already-exhumed metamorphic basement.Alongside the older (0.59–0.58 Ga) detrital rutile population, which dominates the Cambrian sequence, 0.55 Ga detrital rutiles first appear in the Ordovician of Jordan while 0.54 Ga rutile locally dominates the Ordovician from Ethiopia, some 2000 km upstream the Cambro-Ordovician fluvial system, close to its headwaters. The Ordovician drainage basin has thus been extended to include crustal vestiges that were exhumed coeval with Gondwana consolidation, but it is likely that most of the rock carapace that has been eroded from consolidating Cambrian orogenic sutures has been delivered to the south (modern coordinates).     

High grade metamorphism of sedimentary rocks during Palaeozoic rift basin formation in central Australia

Exhumation of middle and lower crustal rocks during the 450–320 Ma intraplate Alice Springs Orogeny in central Australia provides an opportunity to examine the deep burial of sedimentary successions leading to regional high-grade metamorphism. SIMS zircon U–Pb geochronology shows that high-grade metasedimentary units recording lower crustal pressures share a depositional history with unmetamorphosed sedimentary successions in surrounding sedimentary basins. These surrounding basins constitute parts of a large and formerly contiguous intraplate basin that covered much of Neoproterozoic to early Palaeozoic Australia. Within the highly metamorphosed Harts Range Group, metamorphic zircon growth at 480–460 Ma records mid-to-lower crustal (~ 0.9–1.0 GPa) metamorphism. Similarities in detrital zircon age spectra between the Harts Range Group and Late Neoproterozoic–Cambrian sequences in the surrounding Amadeus and Georgina basins imply that the Harts Range Group is a highly metamorphosed equivalent of the same successions. Maximum depositional ages for parts of the Harts Range Group are as low as ~ 520–500 Ma indicating that burial to depths approaching 30 km occurred ~ 20–40 Ma after deposition. Palaeogeographic reconstructions based on well-preserved sedimentary records indicate that throughout the Cambro–Ordovician central Australia was covered by a shallow, gently subsiding epicratonic marine basin, and provide a context for the deep burial of the Harts Range Group. Sedimentation and burial coincided with voluminous mafic magmatism that is absent from the surrounding unmetamorphosed basinal successions, suggesting that the Harts Range Group accumulated in a localised sub-basin associated with sufficient lithospheric extension to generate mantle partial melting. The presently preserved axial extent of this sub-basin is > 200 km. Its width has been modified by subsequent shortening associated with the Alice Springs Orogeny, but must have been > 80 km. Seismic reflection data suggest that the Harts Range Group is preserved within an inverted crustal-scale half graben structure, lending further support to the notion that it accumulated in a discrete sub-basin. Based on palaeogeographic constraints we suggest that burial of the Harts Range Group to lower crustal depths occurred primarily via sediment loading in an exceptionally deep Late Cambrian to Early Ordovician intraplate rift basin. High-temperature Ordovician deformation within the Harts Range Group formed a regional low angle foliation associated with ongoing mafic magmatism that was coeval with deepening of the overlying marine basin, suggesting that metamorphism of the Harts Range Group was associated with ongoing extension. The resulting lower crustal metamorphic terrain is therefore interpreted to represent high-temperature deformation in the lower levels of a deep sedimentary basin during continued basin development. If this model is correct, it indicates that regional-scale moderate- to high-pressure metamorphism of supracrustal rocks need not necessarily reflect compressional thickening of the crust, an assumption commonly made in studies of many metamorphic terrains that lack a palaeogeographic context.     

A rifted tectonic setting for hercynian high-thermal gradient metamorphism in the pyrenees

Hercynian regional metamorphic terrains in the Pyrenees contain evidence of very high-temperature gradients within the crust during metamorphism, with temperatures as high as 700°C attained at 10–12 km below the surface. Stable isotope studies demonstrate that the crust was simultaneously flushed by marine fluids to at least this depth. The absence of any evidence for crustal collision, and the Upper Palaeozoic stratigraphic record for the area, suggest that the tectonic setting for the metamorphism was a zone of continental rifting associated with strike-slip movement. In this zone anatexis occurred at two distinct levels: Cambro-Ordovician pelites at the base of the Palaeozoic sedimentary pile melted to produce per-aluminous magmas, while in the lower Hercynian crust, very large-scale melting generated voluminous granodioritic magmas which then invaded high-structural levels. The thermal structure of the Hercynian crust was profoundly influenced by both convective and advective heat transfer, due to movement of surface derived aqueous fluids, and intrusion of magmas.     

加里东运动构造古地理及滇黔桂盆地的形成--兼论滇黔桂盆地深层油气勘探潜力

中国南方的滇黔桂地区,早古生代与晚古生代之交曾经发生过较为强烈的加里东运动,包括三个幕:寒武纪末期的郁南运动,中、奥陶世末期的都匀运动以及志留纪末期的广西运动;奥陶系与志留系的残留不全和晚奥陶世至志留纪大片古陆———滇黔桂古陆的展布是加里东运动的重要体现。志留纪末期的广西运动之后,在大致相当于早古生代“滇黔桂古陆”分布的地区形成一个特殊的“滇黔桂盆地”,而且在滇黔桂盆地的主体部位常常是泥盆系直接覆盖在寒武系之上。寒武系,特别是下寒武统,由于寒武纪初期的快速海侵作用而在研究区域普遍发育烃源岩系;研究区域的泥盆系,特别是中泥盆统,在台间盆地中发育优质烃源岩。因此,巨大的构造古地理演变和海陆变迁,形成了一个晚古生代的泥盆系优质烃源岩与早古生代的下寒武统优质烃源岩的空间叠合区域,该叠合区域的加里东运动不整合面上、下的储集体即成为该地区的深层油气勘探对象,预示着滇黔桂盆地的深层存在较大的油气勘探潜力。     

Are the Chinese Altai “terranes” the result of juxtaposition of different crustal levels during Late Devonian and Permian orogenesis?

The general structure of the Chinese Altai has been traditionally regarded as being formed by five tectono-stratigraphic ‘terranes’ bounded by large-scale faults. However, numerous detrital zircon studies of the Paleozoic volcano-sedimentary sequences shown that the variably metamorphosed Cambro-Ordovician sequence, known as the Habahe Group, is present at least in four ‘terranes’. It structurally represents deepest rocks unconformably covered by Devonian and Carboniferous sedimentary and volcanic rocks. Calc-alkaline, mostly Devonian, granitoids that intruded all the terranes revealed their syn-subduction related setting. Geochemistry and isotope features of the syn-subduction granitoids have shown that they originated mainly from the melting of youthful sediments derived from an eroded Ordovician arc further north. In contrast, Permian alkaline granitoids, mostly located in the southern part of the Chinese Altai, reflect a post-subduction intraplate setting. The metamorphic evolution of the metasedimentary sequences shows an early MP-MT Barrovian event, followed by two Buchan events: LP-HT mid-Devonian (ca. 400–380 Ma) and UHT-HT Permian (ca. 300–270 Ma) cycles. The Barrovian metamorphism is linked to the formation of a regional sub-horizontal possibly Early Devonian fabric and the burial of the Cambro-Ordovician sequence. The Middle Devonian Buchan type event is related to intrusions of the syn-subduction granitoids during an extensional setting and followed by Late Devonian-Early Carboniferous NE-SW trending upright folding and crustal scale doming during a general NW-SE shortening, responsible for the exhumation of the hot lower crust. The last Permian deformation formed NW-SE trending upright folds and vertical zones of deformation related to the extrusion of migmatites, anatectic granitoids and granulite rocks, and to the intrusions of gabbros and granites along the southern border of the Chinese Altai. Finally, the Permo-Triassic cooling and thrust systems affected the whole mountain range from ca. 265 to 230 Ma. In conclusion, the Chinese Altai represents different crustal levels of the lower, middle and upper orogenic crust of a single Cambro-Ordovician accretionary wedge, heterogeneously affected by the Devonian polyphase metamorphism and deformation followed by the Permian tectono-thermal reworking event related to the collision with the Junggar arc. It is the interference of Devonian and Permian upright folding events that formed vertical boundaries surrounding the variously exhumed and eroded crustal segments. Consequently, these crustal segments should not be regarded as individual suspect terranes.     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

Low-pressure metamorphism of Palaeozoic pelites in the Aspromonte, southern Calabria: constraints for the thermal evolution in the Calabrian crustal cross-section during the Hercynian orogeny

During Hercynian low-pressure/high-temperature metamorphism of Palaeozoic metasediments of the southern Aspromonte (Calabria), a sequence of metamorphic zones at chlorite, biotite, garnet, staurolite–andalusite and sillimanite–muscovite grade was developed. These metasediments represent the upper part of an exposed tilted cross-section through the Hercynian continental crust. P–T information on their metamorphism supplements that already known for the granulite facies lower crust of the section and allows reconstruction of the thermal conditions in the Calabrian crust during the late Hercynian orogenic event. Three foliations formed during deformation of the metasediments. The peak metamorphic assemblages grew mainly syntectonically (S2) during regional metamorphism, but mineral growth outlasted the deformation. This is in accordance with the textural relationships found in the lower part of the same crustal section exposed in the northern Serre. Pressure conditions recorded for the base of the upper crustal metasediments are c. 2.5 kbar and estimated temperatures range from <350 °C in the chlorite zone, increasing to 500 °C in the lower garnet zone, and reaching 620 °C in the sillimanite–muscovite zone. Geothermal gradients for the peak of metamorphism indicate a much higher value for the upper crust (c. 60 °C km^?1) than for the granulite facies lower crust (30–35 °C km^?1). The small temperature difference between the base of the upper crust (620 °C at c. 2.5 kbar) and the top of the lower crust (690 °C at 5.5 kbar) can be explained by intrusions of granitoids into the middle crust, which, in this crustal section, took place synchronously with the regional metamorphism at c. 310– 295 Ma. It is concluded that the thermal structure of the Calabrian crust during the Hercynian orogeny – as it is reflected by peak metamorphic assemblages – was mainly controlled by advective heat input through magmatic intrusions into all levels of the crust.     

Geochemical features,depositional settings,and provenances of Lower Paleozoic rocks in the Mamyn terrane,Central Asian Fold Belt

New data on geochemical features of the Lower Paleozoic terrigenous rocks in the Mamyn terrane (eastern Central Asian Fold Belt) and U–Pb geochronological studies of the detrital zircon from these rocks are presented. The obtained results suggest the following conclusions. 1. At present, the Kosmataya sequence includes different age Lower Cambrian terrigenous–carbonate and Lower Ordovician terrigenous rocks or represents Lower Ordovician olistostromes including limestone blocks with the Lower Cambrian fauna. Lower Ordovician terrigenous rocks were formed in an island arc or active continental margin, mainly, owing to the erosion of Cambrian–Early Ordovician plutons and volcanics that are widespread in structures of the Mamyn terrane and weakly reworked by the chemical weathering. 2. The Silurian Mamyn Formation was developed at a passive continental margin. The main sources of clastic material for this formation were the same Cambrian–Early Ordovician igneous rocks as for the Cambrian sequence, with the participation of Early Silurian and Vendian igneous complexes. The obtained data significantly refine concepts about the geological structure of the Mamyn terrane, which is a member of the Argun Superterrane, one of the largest tectonic structures in the eastern Central Asian Fold Belt.     

The Cambrian to Aquitanian geological record of the Longi-Taormina Unit (Calabria-Peloritani Arc,southern Italy): geodynamic implications

The Longi-Taormina Unit forms the “Dorsale calcaire” of the Peloritani Alpine Belt (southern Calabria-Peloritani Arc). It is made by a thick sedimentary cover of Meso-Cenozoic age overlying a Variscan weakly metamorphosed Cambrian to Carboniferous succession.

The Palaeozoic series consists of pelitic to arenaceous sediments containing layers of acidic and basic volcanics. The acidic volcanics are affected by the “Caledonian” compressional deformations and are referred to Early Ordovician. The basic rocks belong to two different volcanic cycles; the first, not dated, is ascribed to the Caledonian cycle according to its geochemical signature; whereas the second, middle-late Devonian in age, is interpreted to have formed in the framework of pre-Variscan extensional tectonics. During the Variscan Orogeny (330 Ma), the area recorded metamorphism up to subgreenschist-to-greenschist facies and two main deformation phases, marked by syn-schistose early folds (Dv₁), overprinted by dominantly NW-SE trending late folds (Dv₂).

During the Aquitanian, deformation related to the Alpine Orogeny led to imbrication of the Palaeozoic and Meso-Cenozoic series. The sedimentary cover was affected by a series of N090° to N130° trending folds. Detailed stratigraphical and structural investigations on the tectonic contact between the Longi-Taormina Unit, and the overlying Fondachelli Unit indicate that this structure is part of a frontal thrust ramp which developed during the Aquitanian.

Our geological and structural studies on the Cambrian to Aquitanian rocks of the Longi-Taormina Unit of the Calabria-Peloritani Arc enable to unravel the complex geodynamic history of the central-western Mediterranean area.     

从弧后盆地到前陆盆地——北祁连造山带奥陶纪—泥盆纪的沉积盆地与构造演化

北祁连加里东期造山带是在新元古代Rodinia联合大陆(Pangea-850)基础上裂解,经由寒武纪裂谷盆地、奥陶纪初期成熟洋盆、奥陶纪中晚期北祁连活动大陆边缘、志留纪—早、中泥盆世碰撞造山而形成的。奥陶纪中、晚期,北祁连、走廊地区中、上奥陶统发育洋壳-岛弧-弧后火山岩,形成典型的沟-弧-盆体系的沉积。志留纪—早、中泥盆世是北祁连-走廊沉积盆地的转换时期。除天祝、古浪、景泰及肃南等局部地区发育下志留统钙碱性系列火山岩以外,全区志留系均以碎屑岩沉积为主。志留系底部多见一套砾岩层。下—中志留统为典型复理石相的浊流沉积。上志留统变为滨浅海相磨拉石沉积。早、中泥盆世雪山群为典型的陆相粗碎屑磨拉石沉积。从空间分布上看,志留系—泥盆系在走廊—北祁连地区也有自北向南厚度加大、粒度变粗的特征,古流以由南向北、来自造山带的古流为特征。北祁连-河西走廊奥陶纪弧后盆地火山岩—志留系复理石-海相磨拉石—中、下泥盆统陆相磨拉石的充填序列以及空间分布特点,反映为典型的弧后盆地向前陆盆地转化的沉积序列。     

塔里木盆地塔中隆起构造演化与油气关系

笔者结合塔里木盆地形成的构造背景对塔中隆起的主要构造演化过程进行了研究，并具体分析了各构造演化阶段与志留系油气的关系。研究表明，塔中隆起源于早古生代拉张背景下的正断层，寒武-奥陶纪为其烃源岩形成的重要时期；早奥陶世末-晚奥陶世末的构造反转是塔中隆起的主要形成期，同时也为志留系的形成及复合圈闭提供了古地理背景；泥盆纪末塔中鼻状隆起基本定型，古油藏遭受破坏，形成了沥青砂岩；早二叠世晚期的构造变形使古油藏遭受进一步的破坏；二叠纪后构造变动主要以调节为主。伴随整个构造的演化，志留系的油气聚集也是分期次、不同规模的进行。     

Evidence for pre-orogenic,Early Devonian rifting in the Variscan belt: stratigraphy and structure of the Palaeozoic cover of the Mauges Unit (Upper Allochthon,Armorican massif,France)

The Palaeozoic sediments from the Mauges Unit (Armorican Massif, France) are the best-preserved pre-orogenic sequences belonging to the Upper Allochthon. Two coherent sequences are identified. The southern unit (Chateaupanne Unit) represents the cover of the Proterozoic basement and consists of Ordovician sediments unconformably overlain by Emsian carbonates followed by Emsian to earliest Eifelian immature sandstones. The northern unit (Tombeau Leclerc Unit) consists of an Hirnantian to Emsian condensed sequence, in reverse position, that has been thrust over the southern unit. The Devonian unconformity is interpreted as evidence for an Early Devonian extension, recorded by normal faults affecting both the Early Devonian limestones and the underlying Ordovician series. This crustal extension, recorded here for the first time, is possibly related to the opening of a back-arc basin (Saint-Georges-sur-Loire) associated with the subduction of an ocean located further south (Galicia-Brittany-Massif Central Ocean).     

Cambrian to Devonian Geologic Evolution of the Sierra de Comechingones, Eastern Sierras Pampeanas, Argentina: Evidence for the Development and Exhumation of Continental Crust on the Proto-Pacific Margin of Gondwana

Crystalline rocks from the Sierra de Comechingones, eastern Sierras Pampeanas, evolved through three distinct orogenic cycles during the Eopalaeozoic: (1) the first tectono-thermal event named Pampean orogeny (550 to 505 Ma), which peaked in the Early Cambrian, was responsible for extensive metamorphism, partial melting, juvenile magmatism, rapid decompression, and persistent tectonic activity. Large part of the crustal section that was residing at middle levels (c. 27 km) was heated above 800 °C during the thermal peak stage of the Pampean orogeny; decompression of the Pampean orogen's core took place at this high temperature. The exhumation mechanism that assisted rapid uplifting combined the effects of ongoing tectonic forces with a buoyant instability created by a large amount of anatectic magmas in the middle to lower crust. (2) Beginning at the Early Ordovician, the Famatinian orogeny produced an overall shortening, causing pervasive textural reworking of the Cambrian metamorphic sequences under a high-strain regime. By being adjacent to the Famatinian magmatic arc, the western border of the Cambrian crystalline package absorbed imposed deformation along a crustal scale ductile shear zone. Within this zone, the high-grade metamorphic rocks were reworked and re-hydrated to lower temperature assemblages (<600°C and 3–6 kbar). Early Ordovician subduction-related igneous activity, even though manifested as small plutons, intruded Cambrian crystalline sequences, and experienced textural reworking during Late Famatinian tectonic exhumation. Late Famatinian convergence resulted in west-vergent ductile shear zones that placed Cambrian onto Ordovician crystalline sequences. (3) During post-Famatinian times (360–400 Ma) enduring crustal perturbation produced intra-crustal-derived granitic magmatism. West- to northwest-directed thrusting was concentrated in belts nucleated along crustal-scale tectonic boundaries formed between older tectono-stratigraphic units. As a result, Devonian anatectic granites were formed and tectonically extruded among Pampean and Famatinian crystalline sequences. The post-Famatinian event is also characterised by the intrusion of batholith-scale monzogranites into Pampean and Famatinian crystalline sequences residing in the upper crust.

Crystalline rocks currently exposed in the Sierra de Comechingones show that they crystallised and were exhumed in a setting where tectono-thermal activity lasted, even though it might have waned, until the Middle Palaeozoic. From the latest Neoproterozoic (c. 550 Ma) until the Late Devonian (c. 360 Ma) tectonic activity was intermittently acting, indicating continuous convergence along the proto-Pacific margin of Gondwana.     

The Role of Sediment Subduction and Crustal Growth in Hercynian Plutonism: Isotopic and Trace Element Evidence from the Sardinia-Corsica Batholith

The calc-alkaline association of the Hercynian Sardinia-CorsicaBatholith consists of multiple coalescent granitoid plutonsand minor gabbroic complexes. Isotopic and trace element dataare presented for selected gabbros and I-type granitoids representativeof the parental mantle- and crust-derived magmas, respectively.The gabbros belong to normal calc-alkaline suites and have markedrelative enrichments in Rb, Ba, K and Pb in primitive mantle-normalizedtrace element diagrams. The granitoids belong to high-K calc-alkalinesuites and have fairly uniform trace element compositions resemblingvolcanic arc granitoids (VAG). A significant overlap in Sr andNd isotope compositions is observed between gabbros and granitoids. Geochemical and isotopic data provide evidence for the originof the gabbros from mantle sources enriched in incompatibletrace elements through recycling of sediments via subductionzones, whereas the granitoids were derived from crustal sourcescomposed mainly of igneous protoliths with relatively homogeneouscomposition. Sr and Nd isotope compositions of gabbros and granitoidsare consistent with both the mantle enrichment process and theformation of the igneous crustal sources occurring at 450 Ma,during the earlier calc-alkaline igneous activity. The connection between Hercynian and Ordovician igneous activityhas important and new implications for the Palaeozoic evolutionof the Sardinia and Corsica lithosphere, and permits the Hercynianorogeny to be placed in a wider geodynamic setting, consistingof three main phases. The Ordovician precollisional phase wascharacterized by a N-NE-dipping subduction of an oceanic plateunder a continental plate with emplacement of acid and subordinatebasic-intermediate volcanic and intrusive rocks. The subcontinentalmantle underneath Sardinia and Corsica experienced enrichmentin incompatible trace elements through recycling of sediments.Major crustal accretion also occurred with underplating of basalticmagmas. The Devonian collisional phase was characterized bythe collision of two continental plates after the total consumptionof the oceanic plate. Crustal thickening processes took placetogether with regional metamorphic events that recorded a clockwiseP-T-t path. The Carboniferous post-collisional phase was characterizedby isostatic and thermal readjustments following crustal thickeningthat caused extensive partial melting. Large quantities of I-typegranitoids and subordinate gabbroic complexes were emplacedin the middle-upper crust and formed the mainframe of the Sardinia-CorsicaBatholith. This geodynamic model is consistent with the Palaeozoic evolutionof other sectors of Western Europe suggested on the basis ofgeological, geochronological and palaeomagnetic data. The palaeomagneticrestoration of the Late Palaeozoic position of Sardinia andCorsica close to Southern France suggests that Sardinia andCorsica could have been portions of the southern edge of theArmorican plate that, during Siluro-Devonian, collided withthe Ibero-Aquitanian plate after the total consumption of theLate Cambro-Ordovician South Armorican and/or Massif CentralOcean. KEY WORDS: crustal growth; Hercynian orogeny; mantle enrichment; radiogenic isotopes; Sardinia-Corsica Batholith ^*Corresponding author. Present address Dipartimento di Scienza del Suolo e Nutrizione della Pianta, Piazzale Delle Cascine i6, 50144 Firenze, Italy     

内蒙古东七一山花岗岩地球化学、锆石SHRIMP U-Pb年龄及岩体形成环境探讨

古生代时期, 北山地区的地壳活动非常强烈, 主要表现为: 早古生代初期大陆的裂解, 一直到中奥陶世广阔大洋盆的发育。志留纪末, 洋盆在自南向北的俯冲中封闭, 使北侧的哈萨克斯坦板块和南侧的塔里木板块拼贴, 并在碰撞造山过程中又构成了一个相对统一的陆块。在晚古生代, 北山地区地壳又在另外一种形式中异常强烈活动, 特别是自石炭纪到二叠纪, 大规模的中酸性岩浆侵入活动构成本区重要的地质事件, 其出露的花岗岩类占到了全区总面积的近1/3, 但泥盆纪时期的地壳活动, 特别是花岗岩浆的侵入活动常被人们忽视, 笔者据泥盆纪时期的沉积-火山作用及挤压构造活动也较发育认为, 海西早期也应有较强的花岗岩浆侵入活动。本文有针对性地对北山地区, 原定为海西中期的东七一山花岗岩岩基, 在岩石学和地球化学等方面研究基础上, 对3处岩石中锆石首次进行了SHRIMP U-Pb年龄测定, 其结果分别是(355±4) Ma、 (359±4) Ma、(355±5) Ma, 这表明东七一山花岗岩形成于泥盆纪晚期, 从而确定了北山晚古生代早期也有花岗岩浆的强烈活动, 这对深化北山古生代地壳演化过程有积极意义。     

Middle to Late Ordovician arc system in the Kyrgyz Middle Tianshan: From arc-continent collision to subsequent evolution of a Palaeozoic continental margin

New geological, geochronological and isotopic data reveal a previously unknown arc system that evolved south of the Kyrgyz Middle Tianshan (MTS) microcontinent during the Middle and Late Ordovician, 467–444 Ma ago. The two fragments of this magmatic arc are located within the Bozbutau Mountains and the northern Atbashi Range, and a marginal part of the arc, with mixed volcanic and sedimentary rocks, extends north to the Semizsai metamorphic unit of the southern Chatkal Range. A continental basement of the arc, indicated by predominantly felsic volcanic rocks in Bozbutau and Atbashi, is supported by whole-rock Nd- and Hf-in-zircon isotopic data. ε_Nd(t) of + 0.9 to − 2.6 and ε_Hf(t) of + 1.8 to − 6.0 imply melting of Neo- to Mesoproterozoic continental sources with Nd model ages of ca. 0.9 to 1.2 Ga and Hf crustal model ages of ca. 1.2 to 1.7 Ga. In the north, the arc was separated from the MTS microcontinent by an oceanic back-arc basin, represented by the Karaterek ophiolite belt. Our inference of a long-lived Early Palaeozoic arc in the southwestern MTS suggests an oceanic domain between the MTS microcontinent and the Tarim craton in the Middle Ordovician.The time of arc-continent collision is constrained as Late Ordovician at ca. 450 Ma, based on cessation of sedimentation on the MTS microcontinent, the age of an angular unconformity within the Karaterek suture zone, and the age of syncollisional metamorphism and magmatism in the Kassan Metamorphic Complex of the southern Chatkal Range. High-grade amphibolite-facies metamorphism and associated crustal melting in the Kassan Metamorphic Complex restricts the main tectonic activity in the collisional belt to ca. 450 Ma. This interpretation is based on the age of a synkinematic amphibolite-facies granite, intruded into paragneiss during peak metamorphism. A second episode of greenschist- to kyanite–staurolite-facies metamorphism is dated between 450 and 420 Ma, based on the ages of granitoid rocks, subsequently affected or not affected by this metamorphism. The latest episode is recorded by greenschist-facies metamorphism in Silurian sandstones and granodiorites and by retrogression of the older, higher-grade rocks. This may have occurred at the Silurian to Devonian transition and reflects reorganization of a Middle Palaeozoic convergent margin.Late Ordovician collision was followed by initiation of a new continental arc in the southern MTS. This arc was active in the Early Silurian, latest Silurian to Middle Devonian, and Late Carboniferous, whereas during the Givetian through Mississippian (ca. 385–325 Ma) this area was a passive continental margin. These arcs, previously well constrained west of the Talas-Ferghana Fault, continued eastwards into the Naryn and Atbashi areas and probably extended into the Chinese Central Tianshan. The disappearance of a major crustal block with transitional facies on the continental margin and too short a distance between the arc and accretionary complex suggest that plate convergence in the Atbashi sector of the MTS was accompanied by subduction erosion in the Devonian or Early Pennsylvanian. This led to a minimum of 50–70 km of crustal loss and removal of the Ordovician arc as well as the Silurian and Devonian forearcs in the areas east of the Talas-Ferghana Fault.