Mélanges and disrupted rocks at the leading edge of the Humber Arm Allochthon,W. Newfoundland Appalachians: Deformation under high fluid pressure

The Humber Arm Allochthon was structurally emplaced onto the Laurentian margin in western Newfoundland during Taconian (Ordovician) and Acadian (Devonian) deformation. On Port au Port Peninsula, disrupted allochthonous rocks previously mapped as mélange and scaly shale include three mappable, variably disrupted, stratigraphic units; in addition, mixed rocks constitute mélange with much smaller area than previously mapped. At outcrop scale, a qualitative assessment of disruption distinguishes broken, but coherent stratigraphy from a more disrupted and mixed mélange unit. Within coherent regions, three generations of folds are probably related to Taconian, Acadian and Carboniferous deformation events. More disrupted regions show an average of ~24% blocks to 76% matrix with block sizes 0.5–158 cm. A new sampling technique allowed recovery of oriented mélange samples for thin-section. Multiple orientations of extensional fractures suggest approximately coaxial extension. Abundant carbonate and less common bitumen-filled veins suggest that high fluid pressure played a role in the emplacement of the Allochthon. High fluid pressure was probably also responsible for dewatering structures, sandstone dykes and partially brecciated carbonate beds. Map relationships, outcrop and thin-section scale observations lead to a reinterpreted structural history for western Newfoundland in which an early, Taconian, West Bay Thrust Sheet was rapidly emplaced onto the Laurentian margin. During emplacement, debris flows initially contributed igneous blocks to the allochthon, but the majority of fragmentation took place in an environment of horizontal tectonic extension promoted by high fluid-pressure that encouraged brittle fracture. The West Bay thrust sheet was subsequently overridden by the out-of-sequence Lourdes Thrust. Parts of the allochthon were probably re-imbricated in later events, but because of previous disruption, an organized imbricated thrust belt was not developed. At the tip of an advancing thrust wedge, a clear distinction between tectonic and surficial processes of mélange formation may not be possible.     

Contrasting styles of Taconian, Eastern Acadian and Western Acadian metamorphism, central and western New England

The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west-central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back-arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high-T, low-P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E-W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well). The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P–T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400-600° C were reached at pressures of about 4–6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by ⁴⁰Ar/³⁹Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600–725° C) and pressures (6.5–10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite–sillimanite boundary on a P–T diagram. There is little or no plutonism accompanying Western Acadian metamorphism. In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high-T, intermediate-P conditions for the highest grade rocks known in New England: T= 650–750° C, P= 4.5–6.5 kbar for granulite facies assemblages which apparently formed along an ‘anticlockwise’P–T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a ‘clockwise’ path with decompression cooling. Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445 Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe-stage deformation and metamorphism in the Eastern Acadian zone appears to have ended by about 410 Ma. Tectonic thickening in the Western Acadian (including the western counterparts of the nappe-stage deformation documented in the Eastern Acadian) must pre-date attainment of peak metamorphic conditions dated at 395–385 Ma. Dome-stage deformation clearly post-dates peak metamorphism and deforms metamorphic isograds. The end of Western Acadian deformation is well constrained by 370-375 Ma radiometric ages of late pegmatites and granitoids which cross-cut all structures.     

K－Ar测年法在确定沉积岩成岩时代中的应用——以鄂尔多斯盆地为例

根据伊利石Ｋ－Ａｒ测年法研究，确定鄂尔多斯盆地存在两期与构造运动有关的成岩作用事件：即与早期燕山运动有关的侏罗纪（１７０—１６０Ｍａ）和与晚期加里东运动有关的志留－泥盆纪（４２０－３７０Ｍａ）成岩作用时代。为了揭示碎屑物质对Ｋ－Ａｒ年龄的影响和伊利石成岩作用的机制，本文提出了一个通过观察Ｋ－Ａｒ年龄随深度变化的趋势来进行判断的模式。在Ｋ－Ａｒ年龄小于地层时代的条件下，Ｋ－Ａｒ年龄与深度呈正相关，或者很相近的Ｋ－Ａｒ年龄与深度变化无关均标志着没有或很少有碎屑物质的影响；同时，前者指示逐渐埋藏条件下的成岩作用，而后者则反映短暂的热事件引起的成岩作用事件。     

黑龙江省上古生界伊利石的成因标志及其地质意义

采用XRD技术,对黑龙江省上古生界泥质岩石中伊利石结晶度、多型和b值进行了测定,以揭示该区晚古生代极低级变质作用特征。测定结果表明,伊利石结晶度（Kübler指数）主要变化于0.31～0.77 °Δ2θ,伊利石（白云母）b值主要变化于8.983～9.011 A伊利石（白云母）多型为1M_d型、1M_d+2M₁混合型和2M₁型。上述伊利石矿物学参数揭示出,该区上古生界泥质岩石主体属于中-低压相晚期成岩带-低级近变质带范畴。进一步的研究表明,伊利石b值和伊利石结晶度存在明显的负相关关系,即随伊利石b值的逐渐增大（压力升高）,伊利石结晶度逐渐减小（温度升高）,暗示伊利石形成于一种正常的与埋深有关的成岩-变质环境,继而揭示出该区上古生界的成岩作用与变质作用是一个统一的连续过程而不是两个相互无关的地质事件。结合区域地质背景和内蒙、吉林等地伊利石的研究成果可以认为,东北地区上古生界并未遭受大规模的绿片岩相区域变质作用,局部出现的某些绿片岩相（或更高级别）的变质岩应是动力变质作用或接触变质作用的产物。     

Relationships between Precambrian and Lower Palaeozoic rocks of the ‘Avalon Platform’ in New Brunswick, the northeast Appalachians and the British Isles

The Avalon Platform, which is often assumed to be the southeastern margin of the Appalachian—Caledonian orogenic belt, is represented in New Brunswick by the Late Precambrian volcanic rocks of the Coldbrook Group underlain by the metacarbonates and gneisses of the Greenhead Group. The overlying Palaeozoic sequence has been affected by the Acadian (Siluro-Devonian) and Variscan orogenic movements. Granites and a dyke/sill swarm of possible Precambrian age intrude the metasedimentary and volcanic rocks. A pre-Acadian structural event in the Greenhead Group is associated with the local formation of migmatite gneisses. The New Brunswick succession is compared with Cape Breton, Newfoundland and the British Isles. An ensialic volcanic-arc model is proposed for the unified ‘Avalon Platform’ that, during the Late Precambrian, stretched from present-day southern Massachusetts to southern Britain as a microcontinent. The Acadian, Caledonian and Variscan orogenies and the later Mesozoic distentional movements resulted in the fragmentation of the platform.     

Anchimetamorphism of the Neoproterozoic and the Lower Paleozoic along the Profile of Yuanguping in Western Hunan Province, China

The Neoproterozoic and Lower Paleozoic along the profile of Yuanguping in western Hunan Province, China underwent anchimetamorphism. The illite crystallinity (IC) of the <2 μm fractions ranges from 0.23-0.34°△2θfor the Neoproterozoic to 0.23-0.35°△A2θ for the Lower Paleozoic (calibrated with the Kisch IC set, Kisch, 1991). This indicates that the metamorphic grade of the Neoproterozoic and Lower Paleozoic is the anchizone. The peak metamorphic temperature is estimated to be 290-210℃. This result does not agree with the greenschist or subgreenschist facies of the Banxi Group, nor with the lower-greenschist facies or sedimentary cover of the Sinian to Lower Paleozoic, as most previous researchers thought. The illite (K-mica) b0 values range from 0.9074 to 0.8963 (nm) for the Neoproterozoic and the Lower Paleozoic. Based on cumulative frequency curves of the illite (K-mica) b0, the peak metamorphic pressure of the Banxi Group was derived to be of a type that is slightly higher than that of the N. Ne     

Multiple origin and regional significance of bedding parallel veins in a fold and thrust belt: The example of a carbonate slice along the Appalachian structural front

The development and evolution of bedding parallel veins (BPV) are investigated in the Saint-Dominique carbonate slice (southern Québec Appalachian structural front), in order to emphasize the regional significance of BPV in a fold and thrust belt and their importance in establishing its structural and diagenetic evolution. Structural and microstructural analyses reveal that most BPV display a laminated structure locally crosscut by a massive one. Both structures show mutual crosscutting relationships with bedding parallel or bedding perpendicular stylolites and with bedding perpendicular veinlets, indicating that BPV cementation and deformation are continuous processes. Opening both sub-parallel and sub-perpendicular to the BPV walls are documented. Early BPV are planes of weakness that focus later reactivation, and evidence of successive and sometimes opposite senses of opening are locally preserved within a single BPV. The bedding parallel orientation of these veins proved particularly helpful in establishing the structural evolution of an area mostly characterized by front parallel structures with little crosscutting relationships otherwise. Petrographic and geochemical (δ¹⁸O _VPDB and δ¹³C) analyses of structural cements suggest a common, locally derived source for the fluids that percolated through BPV, non-bedding parallel veins and faults. The isotopic ratios also reveal a significant enrichment in ¹⁸O and ¹³C through time. This evolution is tentatively correlated with tectonic, syn-convergence exhumation of the Saint-Dominique slice during its imbrication along the Appalachian structural front, after its burial under Taconian thrust sheets.     

Contrasting red bed diagenesis: the southern and northern margin of the Central European Basin

We compare the diagenetic evolution of deeply buried Rotliegend (Permian) red bed sandstones at the southern and northern margin of the Central European Basin (CEB) in Germany. Main target is to evaluate the influence of maturation products from hydrocarbon (HC) source rocks during red bed diagenesis. At the southern margin of the CEB, thick coal-bearing Carboniferous source rocks are omnipresent beneath the Rotliegend. They contain dominantly gas-prone terrigenous organic material and some oil source rocks. Hydrocarbons were generated from Late Carboniferous onwards throughout most of basin subsidence. At the northern margin of the CEB, source rocks are almost absent due to deep erosion of Carboniferous rocks and a low TOC of local Lower Carboniferous relics. Early diagenetic processes are comparable at both basin margins. Significant differences in burial diagenetic evolution are spatially correlated to the occurrence of hydrocarbon source rocks. Burial diagenesis at the southern margin of the CEB is characterized especially by bleaching of red beds, major dissolution events, pervasive illite formation, impregnation of pore surfaces with bitumen, and formation of late Fe-rich cements. Almost none of these features were detected at the northern basin margin. Instead, relatively early cements are preserved down to maximum burial depths. This suggests that major diagenetic mineral reactions in deeply buried red bed sandstones are controlled by the presence or absence of maturing hydrocarbon source rocks.     

自生伊利石40Ar/39Ar法定年技术及气藏成藏期的确定

为了确定天然气藏的成藏期,进行了自生伊利石的^40Ar／^39Ar法同位素定年实验．实验重点解决了自生伊利石的^40Ar／^39Ar法定年的几个技术难题：第一,粘土矿物的提纯,避免伊利石以外的含K矿物混入;第二,自生伊利石与碎屑伊利石的分离;第三,克服核反冲造成的Ar原子丢失．利用冷冻一加热循环碎样技术获得高纯度的粘土矿物;通过阶段加热得到的年龄谱可以区分自生伊利石与碎屑伊利石;利用“显微包裹”技术有效克服了核照射反冲问题．对鄂尔多斯盆地北部苏里格气田研究发现。二叠系储层中的伊利石有2种年龄图谱：一种只有自生伊利石的坪年龄;另一种图谱既有自生伊利石的坪年龄,也有碎屑伊利石的年龄,形成二阶式的图谱．通过自生伊利石的形成时间推断,天然气的最早充注时间晚于169～189Ma．实验的结果表明,冷冻加热循环碎样技术可以有效地避免伊利石以外的含K矿物混入．是获得高纯度粘土矿物的关键技术;自生伊利石^40Ar／^39Ar法定年技术可以用于确定天然气藏的成藏期.     

Diagenetic trends in the siliceous facies of the Monterey Shale in the Santa Maria region, California

X-ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the Miocene Monterey Shale near Santa Maria, California. Within many stratigraphic sections soft, porous diatomaceous rocks change gradationally to underlying hard and brittle chert, porcellanite, and siliceous shale; the accompanying silica mineral zones are, in descending stratigraphic order: (1) biogenic silica (opal-A), (2) cristobalitic silica (opal-CT), and (3) microcrystalline quartz. Boundaries between silica mineral zones and stratigraphic horizons are often discordant. Within the opal-CT zone, the d(101)-spacing of opal-CT decreases in a smooth non-linear fashion from about 4 10 Å to 4-04 Å. In the Santa Maria Valley and Bradley oil field areas the thicknesses of the opal-CT zones are greater and the present thermal gradients less than in the adjacent Orcutt oil field. Thin opal-CT zones at shallow maximum burial depths apparently correlate with higher thermal gradients. Using present thermal gradients and reconstructed maximum burial depths from well data in the Santa Maria region, the ranges in temperatures for the top and base of the opal-CT zone are 38–54 °C and 55–110 °C, respectively. The temperature difference between these two boundaries ranges from 17 to 60 °C. In comparison, temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal-CT and quartz, extrapolated experimental quartz-water fractionations, and assuming δO¹⁸= 0%_o for the isotopic composition of the equilibrating fluid are 18–56 °C and 31–80 °C for the top and base of the opal-CT zone, respectively. The temperature difference between these boundaries is 11–36 °C using this method. Thermal gradients and sedimentation rates strongly influence rates of silica transformations. Reconstructed thermal and diagenetic histories of siliceous rocks of the Monterey Shale at four well sites in the Santa Maria region demonstrate that most silica conversions probably occurred during the last 3–4 Myr in response to accelerated rates of sedimentation (and therefore burial heating) during the Pliocene.     

Diagenesis of the Lower Cretaceous Kanmon Group sandstones, SW Japan

The Kanmon Group (Lower Cretaceous) is a non-marine sequence in the Inner Zone of southwest Japan and is divided into the lower Wakino (lacustrine) and the upper Shimonoseki (fluvial) subgroups. Major diagenetic changes in this group are compaction, iron-oxide cementation, calcite cementation and grain replacement, quartz overgrowth and pore-fill cementation, illite authigenesis, chlorite pore-fill cementation and grain replacement, albitization of feldspar, and grain replacement by pyrite. Two subgroups of the Kanmon Group present no significant differences in general diagenetic features, paragenetic sequence, or the degree of diagenetic changes despite differences in depositional environments (lacustrine vs. fluvial) and stratigraphic positions. However, some differences are recognized in the content and chemistry of authigenic minerals caused by different sandstone framework compositions. The content of authigenic clay minerals is higher in sandstones of the Shimonoseki Subgroup containing abundant volcanic rock fragments. In addition, the composition of chlorite, the most abundant authigenic clay mineral in Kanmon sandstones, is Mg-rich in the volcanoclastic Shimonoseki sandstones, compared to an Fe-rich variety in Wakino sandstones. The original sandstone composition played a significant role in pore-water composition and diagenetic reactions.The Wakino sandstones lost most of its porosity by compaction, whereas Shimonoseki sandstones are only compacted in the vicinity of the basin-bounding fault. The weakly compacted Shimonoseki sandstones, instead, were largely cemented by pore-filling calcite during early diagenesis; cementation prevented compaction during further burial. The Kanmon Group sediments were heated to about 300 °C based on illite crystallinity values.     

On the timing and causes of illite formation and remagnetization in the Cretaceous Marias River Shale, Disturbed Belt, Montana

In this study, we test a burial model for diagenetic illite and whether illitization is related to the acquisition of a chemical remanent magnetization (CRM). A prefolding, reversed polarity, secondary magnetization is found in carbonate concretions in diagenetically altered Upper Cretaceous rocks in the disturbed belt of Montana. Stratigraphically equivalent unaltered rocks to the east do not contain the magnetization. The magnetization could be a CRM or a thermoviscous remagnetization. The pole position is generally consistent with the Early Tertiary K–Ar ages of diagenetic illite–smectite formed under thrust loading, but it is not constrained well enough for a precise age comparison. If this magnetization is a CRM, it could be related to illitization. Alternatively, if the prefolding CRM had been acquired prior to thrust loading then the magnetization is not related to illitization. Geochemical studies are currently underway to test if the magnetization could be related to fluid alteration.     

Tectono-thermal evolution in a region with thin-skinned tectonics: the western nappes in the Cantabrian Zone (Variscan belt of NW Spain)

 The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC). Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role of the burial on the metamorphism. Received: 11 May 1998 / Accepted: 19 January 1999     

沉积岩中成岩伊利石年龄测定研究进展

伊利石的形成与盆地的沉积和热演化有密切关系，它可以指示油气的形成时代，因此测定成岩伊利石的形成年龄是一个重要的课题。碎屑伊利石往往与成岩伊利石共生，导致样品的年龄远远大于成岩伊利石的真实年龄。为了有效地分离成岩和碎屑伊利石，需要使用IAA法和烷基胺阳离子取代法等，结合真空密封法等其他技术，能够得到更好的效果。并介绍了国外伊利石定年的研究现状。     

Deposition and diagenesis of carbonate conglomerates in the Kremenara anticline, Albania: a paragenetic time marker in the Albanian foreland fold-and-thrust belt

This paper addresses the diagenesis of carbonate conglomerates in that it assesses the potential of conglomerates in refining the paragenetic history in complex structural areas, such as the Albanian foreland fold‐and‐thrust belt. Of major interest are stylolites (burial and tectonic) which are restricted to conglomerate fragments or which crosscut the conglomerate matrix. Based on the inferred age of stylolite development in relation to burial, uplift and tectonic history, and the Lower to Middle Miocene age of the conglomerates, the succession of diagenetic events was subdivided into several stages. The Poçem polymict transgressive carbonate conglomerate (Kremenara anticline, central Albania) was deposited in a shallow marine environment. These conglomerates are covered by intertidal rhodolithic packstones–grainstones. The stable‐isotope signature of these packstones–grainstones (δ¹⁸O_V‐PDB = −1·0 to +0·7‰; δ¹³C = +1·0 to +1·4‰) plots is within the range of marine Early and Middle Miocene values. Shortly after deposition of the conglomerates, micritization, geopetal infill and acicular calcite cementation took place. A first calcite vein generation is interpreted as having formed from a Messinian brine during shallow burial. Burial stylolites developed during further burial in the Pliocene. These stylolites serve as an important diagenetic time marker. The post‐burial stylolite meteoric calcite vein cement probably precipitated during the following telogenetic stage. Karstification and calcite concretion precipitiation pre‐date overturning of the western limb of the anticline. Reopening of subvertical fractures and tectonic stylolites in the western limb of the Kremenara anticline, followed by oil migration, represents one of the latest diagenetic events. These fractures and stylolites provide major pathways for hydrocarbon production.     

塔里木盆地西北部寒武系纽芬兰统玉尔吐斯组混积岩早成岩期岩溶特征及其地质意义

通过塔里木盆地西北部阿克苏-柯坪-乌什地区露头剖面的宏微观工作发现,寒武系纽芬兰统玉尔吐斯组存在3期稳定发育的古暴露面,并具有(1)铁质结壳层、(2)近地表喀斯特(塑形)角砾、(3)溶沟、溶缝及囊状溶洞、(4)岩溶系统内的角砾、陆源碎屑充填物、(5)暴露面之上的地层超覆等暴露标志。综合分析认为研究区玉尔吐斯组并非连续沉积,内部存在多个沉积间断,并与肖尔布拉克组为区域性不整合接触。同时,在与灰岩和白云岩早成岩期岩溶特征对比分析的基础上,指出先期渗滤条件较差的混积岩(硅质碎屑-碳酸盐混合沉积)早成岩期岩溶以充填程度较高的溶沟、溶缝和小型溶洞发育为特征,且水平潜流岩溶带发育不成熟;以此区别于灰岩和云岩型早成岩期岩溶发育的相控溶蚀孔洞、岩溶花斑和近原地角砾化等典型特征。研究结果表明开放条件下的早成岩期暴露岩溶特征及储集层优化与否更多受岩石组构特征控制,由此推断在相对封闭的埋藏条件下,先期致密化的岩石难以经历岩溶改造形成相控的孔洞型储集层。这一认识不仅对深埋条件下储集层的形成和保持具有重要的借鉴意义,也为早成岩期岩溶地质理论的丰富和发展提供了新的材料。     

Dating slate belts using 40Ar/39Ar geochronology and zircon ages from crosscutting plutons: A case study from east-central Maine,USA

We report the ages of cleavage development in a normally intractable lower greenschist facies slate belt, the Central Maine-Aroostook-Matapedia belt in east-central Maine. We have attacked this problem by identifying the minimum ages of muscovite in a regional Acadian cleavage (S₁) and in a local ductile fault zone cleavage (S₂) using ⁴⁰Ar/³⁹Ar geochronology and the ages of crosscutting plutons. Our success stems from the regional low-grade metamorphism of the rocks in which each crystallization event preserves a⁴⁰Ar/³⁹Ar crystallization age and not a cooling age. Evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital, authigenic, and in some rocks S₁ muscovite and chlorite grains by new cleavage-forming muscovite and chlorite grains. Low-blank furnace age spectra from meta-arkosic and slaty rocks climb from moderate temperature Devonian age-steps dominated by cleavage-forming muscovite to Ordovician age-steps dominated by a detrital muscovite component. S₁- and S₂-cleaved rocks were hornfelsed by granitoids of ∼407 and 377 Ma, respectively. The combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage-forming events, ∼410 Ma (S₁) and ∼380 Ma (S₂). These two events coincide in time with a change in the plate convergence kinematics from the arrival of the Avalon terrane (Acadian orogeny), to a right-lateral transpression arrival of the Meguma terrane in the Neoacadian orogeny.     

LA-ICP MS Zircon U-Pb Dating of Metasedimentary Rocks in Dabie Orogenic Belt and Its Tectonic Implications

Low-grade metamorphic rocks concentrated in the Beihuaiyang unit and were sporadically in other units of the Dabie orogenic belt, whose coexistence with other rocks in the orogenic belt can provide evidence for the evolution of the Dabie orogenic belt. A small number of metasedimentary rocks crop out in the Shenqiao area, northeast of the North Dabie unit. In order to obtain the forming age and provenance of the low-grade metamorphic rocks and their relationship with the rocks in the North Dabie unit, detailed field observations, systematic microscopic identification and detrital zircon LA-ICP MS U-Pb dating were presented for metasedimentary rocks in the Shenqiao area. The results show that metasedimentary rocks in the Shenqiao area have similar petrological characteristics as low-grade metamorphic rocks of the Zhufoan Formation in the Foziling Group. The minimum age of detrital zircon is 431 Ma, which is also similar as the Foziling Group. All these reveal that these low-grade metamorphic rocks in the Shenqiao area may be a part of the Foziling Group. Detailed observations on thin-sections show that quartz are characterized by Subgrain Rotation (SGR) recrystallization and biotite has obvious growth rims, which indicates that the metasedimentary rocks experience an epidote-amphibolite facies metamorphic and deformation event. The rhythmic layers on the outcrop existed before experiencing low-grade metamorphism, during which the argillaceous layer in the protolith transferred to the darker layered biotite and the sandy layer to lighter and recrystallized quartz.     

Sedimentary facies and diagenetic features of the Early Cretaceous Fahliyan Formation in the Zagros Fold-Thrust Belt,Iran

The Early Cretaceous Fahliyan Formation (middle part of the Khami Group), is one of the important reservoir rocks in the Zagros Fold-Thrust Belt. The Zagros Fold-Thrust Belt is located on the boundary between the Arabian and Eurasian lithospheric plates and formed from collision between Eurasia and advancing Arabia during the Cenozoic. In this study area, the Fahliyan Formation with a thickness of 325 m, consists of carbonate rocks (limestone and dolomite). This formation overlies the Late Jurassic Surmeh Formation unconformably and underlies the Early Cretaceous Gadvan Formation conformably at Gadvan Anticline. The formation was investigated by a detailed petrographic analysis to clarify the depositional facies, sedimentary environments and diagenetic features in the Gadvan Anticline. Petrographic studies led to recognition of the 12 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid-ramp environments. The absence of turbidite deposits, reefal facies, and gradual facies changes show that the Fahliyan Formation was deposited on a carbonate ramp. Calcareous algae and benthic foraminifera are abundant in the shallow marine carbonates of the Fahliyan Formation. The diagenetic settings favored productioning a variety of features which include cements from early to late marine cements, micritization, dolomitization, compaction features, dissolution fabric, and pores. The diagenetic sequence can be roughly divided into three stages: (1) eugenic stage: marine diagenetic environment, (2) mesogenic stage: burial environment, and (3) telogenic stage: meteoric diagenetic environment.