Breccias in the Lala IOCG Deposit, SW China

Please?refer?to?the?attachment(s)?for?more?details.     

Carboniferous subvolcanic activity on the Beara Peninsula,SW Ireland

Magmatic activity associated with the Munster Basin has been more widespread than previously reported. The Munster Basin is a substantial sedimentary basin, and towards the end of its extensional phase of development, at the beginning of the Variscan orogeny in Ireland, numerous intrusions were emplaced into consolidated Upper Devonian and Lower Carboniferous sediments on the Beara Peninsula. One hundred and sixty-four sills and dykes have been mapped which are subalkaline to alkaline in nature. Two separate suites have been identified. The northern suite comprises subalkaline basalts of Cod's Head and Dursey Island which are intruded into Devonian Red Beds, and the southern suite comprises alkali basalts, trachytes and phonolites which crop out along 9 km of the south coast of the Beara Peninsula and are suggested as Brigantian in age. They are intruded into Devonian Red Beds and marine Lower Carboniferous strata and are therefore later than the tholeiitic magmatism on the Iveragh peninsula to the north. The alkaline magmatism on Beara was induced by lithospheric thinning and controlled partly by pre-existing zones of weakness in the Caledonide crust and partly by fracture zones that developed parallel to the Munster Basin margin as it subsided. In contrast to the Iveragh Peninsula, the stretching factor for the Beara lithosphere was never large enough to lead to the production of tholeiitic magmas. © 1997 John Wiley & Sons, Ltd.     

The geochemistry and petrogenesis of the Lower Paleozoic granitoids of the Lleyn Peninsula,North Wales

The Lleyn Peninsula of North Wales is penetrated by at least twenty stocks varying from microtonalite through to microgranite and these are closely associated with Caradocian volcanic rocks which include andesitic lavas.Major and trace element analyses show that the sample population may be separated into a peralkaline group and a sub-alkaline group. This latter group may be sub-divided into three compositional series which range from andesite to granite. The andesites are represented by the Caradocian lavas of the Moel y Penmaen district and these plot at one end of major and trace element variation diagrams. Their coherent relationships to the granitoids demonstrate the latter to be Ordovician and therefore ends a longstanding controversy concerning their age. The distinctive geochemistry of the andesites indicates the strong probability that they have evolved from transitional tholeiitic magma by low-pressure crystal fractionation. Apatite fractionation has strongly controlled the rare earth element abundances in the granitoids and for most compositions from andesite through granite their bulk distribution coefficient is greater than unity.The association of peralkaline granites, transitional tholeiites and other geological and geophysical evidence suggests that the area was a region of ensialic crustal tension during the mid-Ordovician.     

Geophysical survey of Precambrian terranes in SW Lleyn,Caernarvonshire, Wales

The geology of southwest Lleyn comprises two Late Precambrian terranes: the Gwna Mélange in the west and the Sarn Complex in the east, separated by the Lleyn Shear Zone. The location of the terrane boundary is poorly constrained due to the limited exposure. We undertook a ground magnetic survey and also measured four gravity profiles with the original intention of investigating the cause of a positive aeromagnetic anomaly previously recorded close to the terrane boundary at grid reference [SH 2200 3000]. This ‘Sarn anomaly’ appears to be associated with a shallow body within the Sarn Complex, which is known to be a heterogeneous plutonic igneous unit. Of greater interest was the identification of a much larger-amplitude magnetic anomaly striking roughly N–S for over 7 km close to the mapped position of the Lleyn Shear Zone. It can be modelled as a near-vertical discontinuous body of overall dyke-like form, and is too narrow to have been resolved by the aeromagnetic survey. We discovered a previously unreported outcrop of gabbro at Brynhunog Bach [2100 3127] located on this high-amplitude ‘Brynhunog anomaly’. It seems likely that the whole anomaly is due to a gabbroic body which is an original constituent of the Sarn Complex, but an alternative possibility is that it is a later intrusion along the Lleyn Shear Zone. © 1997 John Wiley & Sons, Ltd.     

Surface effects of triggered fault slip on Reykjanes Peninsula, SW Iceland

Three large earthquakes (M_w>4.5) were triggered within 5 min, 85 km west of a M_w 6.5 earthquake in the South Iceland Seismic Zone (SISZ). We report on surface effects of these triggered earthquakes, which include fresh rupture, widespread rockfall, disrupted rockslides and block slides. Field data confirm that the earthquakes occurred along N-striking right-lateral strike-slip faults. Field data also support the conclusion from modeling of InSAR data that deformation from the second triggered event was more significant than for the other two. A major hydrological effect was the draining of water through an open fissure on a lake bed, lowering the lake level by greater than 4 m. Field relationships suggest that a component of aseismic slip could have been facilitated by water draining into the fault zone.     

The Late Devensian glaciation of the eastern Lleyn Peninsula,North Wales: evidence for terrestrial depositional environments

The glacial deposits of the eastern Lleyn Peninsula record the advance, coalescence and subsequent retreat and uncoupling of Welsh and Irish Sea ice-sheets during the Late Devensian cold stage. During advance a thick sheet of basal diamict was deposited over much of the area, and during retreat and uncoupling, which occurred after 14.5 ka, the eastern part of the area was dominated by the formation of a large sandur system draining from the retreating margin of the Irish Sea ice-sheet. Subsequent stages of retreat are marked by a series of arcuate cross-valley moraines formed either by ice-contact deposition during stillstand or by structural deformation during minor snout oscillation. In the western part of the area sedimentation was controlled by a series of dead-ice ridges running parallel to the retreating ice-margin and this led to the development of a complex assemblage of localised depositional environments including ice-front alluvial fans, marginal sandur troughs and pro-glacial lake basins, all formed under supraglacial ice-marginal conditions. No evidence of glaciomarine deposition is recorded. © 1998 John Wiley & Sons, Ltd.     

The relative age of some concretions in Cambrian sediments of St. Tudwal's Peninsula,North Wales

A new classification of concretions is suggested. Some concretions are described from the Upper Cambrian (Maentwrog) sediments of St. Tudwal's Peninsula, southwest Caernarvonshire, and their age relative to that of the surrounding sediments is discussed.     

The transition from an active to a passive margin (SW end of the South Shetland Trench, Antarctic Peninsula)

The lateral ending of the South Shetland Trench is analysed on the basis of swath bathymetry and multichannel seismic profiles in order to establish the tectonic and stratigraphic features of the transition from an northeastward active to a southwestward passive margin style. This trench is associated with a lithospheric-scale thrust accommodating the internal deformation in the Antarctic Plate and its lateral end represents the tip-line of this thrust. The evolutionary model deduced from the structures and the stratigraphic record includes a first stage with a compressional deformation, predating the end of the subduction in the southwestern part of the study area that produced reverse faults in the oceanic crust during the Tortonian. The second stage occurred during the Messinian and includes distributed compressional deformation around the tip-line of the basal detachment, originating a high at the base of the slope and the collapse of the now inactive accretionary prism of the passive margin. The initial subduction of the high at the base of the slope induced the deformation of the accretionary prism and the formation of another high in the shelf—the Shelf Transition High. The third stage, from the Early Pliocene to the present-day, includes the active compressional deformation of the shelf and the base-of-slope around the tip-line of the basal detachment, while extensional deformations are active in the outer swell of the trench.     

Geochemical Constraints from Zoned Hydrothermal Tourmalines on Fluid Evolution and Sn Mineralization: an Example from Fault Breccias at Roche, SW England

Hydrothermal fluid evolution north of the St Austell granite,southwest England, has been studied through geochemical analysisof tourmaline from a fault breccia of <2 cm width withinmassive quartz–tourmaline rocks at Roche. Brecciated tourmalinegrains have overgrowths of <400 µm width [Fe/(Fe +Mg) = 0·31–0·99] with four chemically distinctzones (1–4, towards the margins). Variations in overgrowthcomposition were caused by episodic mixing between Mg-, Al-richmagmatic hydrothermal fluids (dominant in zone 1), with an increasingcomponent of more oxidizing, Fe-rich formation waters (zones2 and 4). More oxidizing conditions are supported by high Sncontents in zone 2 (<0·35 wt %), with Sn probablypresent as Sn⁴⁺ rather than Sn²⁺, the usual form in hydrothermalfluids. From X-ray maps, zones 1 and 3 occur exclusively asovergrowths on pre-existing grains, indicating that overgrowthformation was kinetically favoured over tourmaline nucleation.In zones 2 and 4, nucleation and growth occurred, possibly asa result of supersaturation with respect to tourmaline duringincreased mixing with formation waters. Tourmaline is associatedwith the main episode of mineralization in many important mineraldeposits, often unaffected by alteration. This method of studyinghydrothermal fluid evolution may therefore have uses in exploration,particularly for tourmaline-breccia-hosted ores in Cu-porphyrydeposits. KEY WORDS: breccia; Cornwall; hydrothermal; tin; tourmaline     

论界牌峪地区“陀状”角砾岩成因及找矿意义

湖南石门界牌峪地区存在大量的所谓“陀状”角砾岩，它们互不相连，外观圆形，有的聚集成群或者呈完全孤立的山包。角砾岩地表多为椭圆状和等轴状，与围岩多呈刺穿状接触。通过地质学、岩石学和地球化学等方面研究，认为该区“陀状”角砾岩是一种由热泉气体及部分物质在封闭还原条件下产生急剧爆发所形成的特殊的角砾岩群。它们是一种极好的容矿控矿体，具有重要的找矿意义     

Mountain glaciation and paleoclimate reconstruction in the Picos de Europa (Iberian Peninsula,SW Europe)

Geomorphic mapping and stratigraphic analysis of a lake core document the late Quaternary glacial history of the Central and Eastern Massifs of the Picos de Europa, northwestern Spain. The distribution of glacial deposits indicates that at their most advanced positions glaciers occupied 9.1 km², extended as far as 7 km down-valley and had an estimated equilibrium-line altitude (ELA) ranging between 1666 and 1722 m. Radiocarbon dating of sediment deposited in a lake dammed by moraines of this advance show that the maximum glacial extent was prior to 35,280 ± 440 cal yr BP. This advance was followed by two subsequent but less extensive late Pleistocene advances, recorded by multiple moraines flanking both massifs and sedimentary characteristics in the lake deposits. The last recognized glacial episode is the 19th-century maximum extent of small Little Ice Age glaciers in the highest cirques above 2200 m.     

Ashgill trilobite faunas from the Llŷn Peninsula,North Wales,U.K.

The Crugan Mudstone Formation is formalized and the Dwyfor Mudstone Formation proposed for restricted Ashgill outcrops respectively near Llanbedrog and Llanystwmdwy in the Llŷn Peninsula. These mudstones contain brachiopods of the Foliomena fauna and a total of 26 determined genera of trilobites, one, Pseudosphaeroxochus represented by a new species, P. seabornei sp. nov. Elements of the trilobite fauna are used to demonstrate a Rawtheyan age for the mudstones. The generic assemblages both of brachiopods and of trilobites present in the mudstones indicate relatively deep water environments of deposition, that of the Dwyfor Mudstones probably being the deeper. The trilobites belong to what is here described as the Nankinolithus-Opsimasaphus association.     

Evidence from the Mesta half-graben, SW Bulgaria, for the Late Eocene beginning of Aegean extension in the Central Balkan Peninsula

The ENE-tilted Mesta half-graben contains a 3-km-thick section of Priabonian (Late Eocene) to Oligocene sedimentary and volcanic rocks that rest unconformably on basement metamorphic rocks along its west side. Basal strata dip 50–60° E and dip at progressively lower angles upward, indicating synrotational deposition. The southern part of the half-graben contains nested volcanic caldera complexes, formed during the deposition of the middle part of the sedimentary sequence, which have been rotated by about half the total rotation of the sedimentary succession. The half-graben is bounded on the east by a fault that steepens from more deeply exposed structural levels in the south (8–18° W) to shallower exposed structural levels in the north (70° W) and together with the rotation of Paleogene strata during deposition indicate the Mesta half-graben is underlain by a listric detachment fault, the Mesta detachment. Subhorizontal Middle Miocene strata that unconformably overlie tilted Paleogene strata yield an upper age limit to the extension. West and northwest of the Mesta half-graben are many other NNW-trending NE-tilted Paleogene half-grabens which we suggest are part of an important extended area in SW Bulgaria and eastern Macedonia that lies above one or more west-dipping detachment faults and date the beginning of Aegean extension in the southern Balkan region as at least as old as Priabonian. The Mesta detachment is oblique to the trend of a contemporaneous Paleogene magmatic arc in the southern Balkans and the origin of the detachment is probably related to gravitationally induced spreading of thickened hot arc crust and Hellenic trench roll back.     

Incipient metamorphism and deformation in the Variscides of SW Dyfed, Wales: first steps towards isotopic equilibrium

Incipient metamorphism accompanying thrusting, folding and cleavage development has been investigated in a varied sequence of Palaeozoic sediments near the Variscan front in SW Dyfed, Wales. The aim was to evaluate a critical stage in the progression from heterogeneous sediment, whose detrital phases are neither in equilibrium with one another, nor with pore fluids, through indurated sedimentary rock to metamorphic rock comprising newly formed crystals that equilibrated with one another as they grew. Quartz veins are widely developed in the area, especially in the more psammitic lithologies, while finer grained rocks became cleaved during tectonic deformation. Mineralogical constraints and fluid inclusion measurements suggest maximum temperatures around 200-310d? C (slightly higher in the Marloes-Musselwick Thrust Sheet than in other parts of the structural succession) at depths of the order of 6-13 km. Quartz veins yield distinctly heavier oxygen isotopic compositions than detrital quartz grains in the adjacent wall rocks, although care must be taken in interpreting the data because slivers of detrital grains may become incorporated into veins, while matrix detrital grains may incorporate veinlets or rims of newly formed quartz. It is concluded that vein quartz grew in isotopic equilibrium with a fluid phase whose isotopic composition was primarily controlled by exchange with phyllosilicates, not detrital quartz grains. Vein and matrix quartzes from the Marloes-Musselwick Thrust Sheet are distinctly lighter (δ¹⁸O_veins=+14 to +18% and δ¹⁸O_matrix=+11 to +14%) than those from other thrust sheets (δ¹⁸O =+17 to +20% and +14 to +17%, respectively). We conclude that vein quartz and phyllosilicate grains in cleavage domains probably attained equilibrium with a locally buffered pore fluid at the peak of metamorphism, but many relict grains of different chemical and isotopic composition remained elsewhere in the rock. Local fluid migration along veins and through cleavage lamellae facilitated the attainment of equilibrium, but there is little evidence for large-scale infiltration of externally derived fluids. With further metamorphism the quartz in these rocks would attain an isotopic composition intermediate between that of the heavy vein material and light detritus which coexist here.     

Palaeocurrent determinations by magnetic fabric measurements on the Cambrian Rocks of St. Tudwal's Peninsula,North Wales

The relationship between depositional grain orientation and current direction is discussed, and a method to assist in distinguishing between depositional and deformational magnetic fabrics is suggested. Magnetic fabric measurements from Middle and Upper Cambrian strata at St. Tudwal's Peninsula are presented. In the Maentwrog Beds (Upper Cambrian) a correspondence is found between the trend of the maximum magnetic susceptibility axes and the trend of the palaeocurrent as deduced from sole markings. In the Caered Mudstones and Flags (Middle Cambrian) the directions of maximum magnetic susceptibility are used to indicate palaeocurrent trends.     

Geochemistry of an ordovician basalt-trachybasalt-subalkaline/peralkaline rhyolite association from the Lleyn Peninsula,North Wales,U.K

The Caradoc volcanic rocks of the Lleyn Peninsula represent a major episode of rhyolite-dominated eruption (total volume up to 560 km³) within the Southern British Caledonides. New geochemical data reveal the original chemical characteristics of the basic, intermediate, and acid lavas. The basalts are interpreted as having predominantly within-plate character and are associated with intermediate lavas (trachybasalts) and high-Zr, originally peralkaline, trachytes, and rhyolites. A low-Zr, originally high-K, subalkalic group of rhyolites, and a basalt with transitional island arc tholeiite-MORB characteristics are unrelated to these lavas. The volcanic rocks are interpreted to have been erupted onto a complex, rapidly evolving, active continental margin dominated by crustal tension.