Precambrian Age Research Articles

Detrital zircon U–Pb and Hf isotopic data for meta-sedimentary rocks from the Heilongjiang Complex, northeastern China and tectonic implications

The Heilongjiang Complex is a blueschist facies metamorphic belt located within the Zhangguangcailing Orogen between the Jiamusi and Songliao blocks in Northeast China. This complex has been regarded as an accretionary belt related to the subduction of an intervening oceanic domain between the two blocks. However, the timing of ocean closure and final amalgamation has not been well constrained, with different models arguing for a period of 210–180Ma or sometime after ~140Ma. This work reports in-situ detrital zircon U–Pb and Hf isotopic analyses of meta-sedimentary rocks from the Heilongjiang Complex. Detrital zircons from seven meta-sedimentary rocks samples yield U–Pb ages spanning from 1690 to 167Ma, with main populations matching those of multi-phase magmatism in the Jiamusi and Songliao blocks. Several Precambrian age groups (600Ma, 700Ma, 900Ma, 960Ma, 1200Ma, and 1300Ma) are consistent with the inherited zircons from the mafic rocks in the Heilongjiang Complex. A comparison with compiled data of magmatic rocks suggests that the two blocks may have been connected to each other during Permian time. Detrital zircon dating of all siliciclastic rocks yielded the youngest age component of ~170Ma, suggesting that the latest deposition of the mica schists happened at some time after ~170Ma. We propose that the Jiamusi and Songliao blocks once existed as a single block around Late Permian, which underwent a rifting event in the Permian to form a rifting basin that was subsequently evolved into an oceanic domain (Heilongjiang Ocean). The closure of the Heilongjiang Ocean occurred after ~170Ma.

Mantle transition zone-derived EM1 component beneath NE China: Geochemical evidence from Cenozoic potassic basalts

The isotopic characteristics of the sub-oceanic mantle are well established, but in continental regions these properties are usually obscured, and therefore controversial, because of the potential effects of crustal contamination together with lithospheric mantle metasomatism and melting. The so-called EM1 (Enriched Mantle-1) signature, characterized by low 206Pb/204Pb and 143Nd/144Nd ratios, is particularly problematic in this respect because EM1-type OIB sources are commonly attributed to recycled crust and/or lithospheric mantle. In this paper we show that a suite of Cenozoic potassic basalts from NE China displays many previously unrecognized correlations between chemical and isotopic parameters, which tightly constrain the isotopic characteristics of an extreme EM1-type mantle source located in the asthenosphere. Its radiogenic isotopes are similar to, but even more extreme than, those of the oceanic endmember composition represented by the Pitcairn hotspot, namely 206Pb/204Pb ≤ 16.5, 143Nd/144Nd ≤ 0.5123 (or εNd≤−6.4), 176Hf/177Hf ≤ 0.2825 (or εHf≤−10.1). These characteristics require a source of recycled crustal material of Precambrian age (∼2.2 Ga). An important new constraint is the Mg isotopic composition of δ26Mg (≤−0.6‰), which is lower than normal mantle (δ26Mg=−0.25±0.07‰) and lower crustal values (δ26Mg=−0.29±0.15‰), but consistent with sedimentary carbonate (δ26Mg=−5.57‰ to −0.38‰). The endmember EM1 source produced high-SiO2 melts with low MgO, CaO/Al2O3 and δ26Mg values, exceptionally high K/U ≅ 50,000, Ba/Th ≅ 400, low U/Pb ≅ 0.06, and positive Zr and Hf anomalies. The chemical and isotopic parameters of this potassic basalt suite form binary mixing arrays, one end point of which is the inferred asthenospheric EM1 reservoir, whereas the other is a more ordinary, depleted mantle component, which is also sampled by local lithospheric mantle xenoliths. These binary arrays include well-developed correlations between Sr, Nd, Hf, Pb and Mg isotopes, negative correlations of 206Pb/204Pb with K2O, K/U, Hf/Hf⁎, positive correlations of δ26Mg with MgO, and 143Nd/144Nd with Fe2O3T and U/Pb.We propose that the EM1 reservoir contains recycled ancient carbonate-bearing sediments, subducted into the mantle transition zone, where K, Rb, Ba and Pb are sequestered by K-hollandite as suggested by Murphy et al. (2002) for the Gaussberg lamproites. Loss of small amounts of carbonate melt extracted Th, U and some of the LREE, while retaining K, Rb, Ba, Pb, Zr and Hf in the residue, thereby generating the observed trace element anomalies. In Cenozoic time, this deep EM1 reservoir ascended into the shallow asthenosphere and underwent low-degree partial melting, at pressures below the stability field of K-hollandite, thereby releasing K, Rb and Ba into the melt. The partial melts ascended through subcontinental lithosphere and were progressively modified by interaction with the lithospheric mantle, thus accounting for the linear chemical and isotopic trends noted above. This interaction imposed a progressively more depleted character on the erupted melt, both in terms of isotopic composition and trace element enrichment.

Chronological constraints on the tectonic evolution of the Chinese Tianshan Orogen through detrital zircons from modern and palaeo-river sands

ABSTRACTThe Chinese Tianshan Orogen marks prolonged and complicated interactions between the southwestern Palaeo-Asian Ocean and surrounding blocks. New and previously published detrital zircon chronological data from modern and palaeo-river sands were compiled to reveal its tectonic evolution. It is characterized by predominant Palaeozoic as well as minor Mesozoic and Precambrian detrital zircon ages with a multimodal characteristic. The oldest Phanerozoic zircon population (peaking at 475 Ma) is a result of subduction and closure of the early Palaeozoic Terskey Ocean. However, the absence of this peak in the Chinese North and southern South Tianshan suggests that subductions of the North and South Tianshan oceans may not have initiated until the Late Ordovician with subsequent 460–390 and 360–320 Ma arc magmatism. Similar to the magmatic suite in classic collisional orogens, the youngest massive 320–270 Ma magmatism is suggested to be post-collisional. The North and South Tianshan oceans therefore probably had their closure to form the Chinese Tianshan Orogen during the late Carboniferous. The weak Mesozoic intra-plate magmatism further rejects a late Permian–Triassic Tianshan Orogen due to a lack of extensive syn- and post-collisional magmatism. Moreover, diverse Precambrian detrital zircon age patterns indicate that the surrounding blocks have distinct evolutionary processes with short-term amalgamation during the Meso- to Neoproterozoic.

THE COMPARATIVE ANALYSIS OF PRECAMBRIAN GOLD RESERVES FORMATIONS OF CRYSTALLIZED SHIELDS

Gold reserves localized in Precambrian deposits are known in the most crystallized shields of the Earth. They are confined to Greenstone belts on the Canadian, Indian, Brazilian and Australian shields and represented by gold reserves deposits from the first ton to 1000 – 1500 tons (Pickle crow, nor-Acme, Homestake, etc. in North America, Kalgoorlie, Leonora and others in Western Australia). Despite the presence of trough structures of Precambrian age, some of which can be considered as analogues of Greenstone belts, the Aldan shield is quite specific of the named above. The comparison of ore fields help evaluate the genesis process of gold mineralization localized in Precambrian formations crystalline shields of the planet and thus define their similarity. There is a number of features that let evaluate the uniqueness of similarities: the anomaly of the magnetic field in the plan coincide with the gravity anomalies indicating the presence of dome uplifts of the basement; the rocks that led to the existence of anomalies of the magnetic field are similar in their geochemical features; the structures mapped by these anomalies are similar to the narrow anticlinal folds; the age of those structures is approximately the same; the identity of the ore-controlling factors and stratigraphic positions. These features allow to accurately assess the similarity of deposit named after P. Pinigin that has the largest gold resources in the world. Therefore, it gives the opportunity to hope for a significant expansion of the gold potential of the Precambrian age at the Aldan shield.

Structural Study and Analytical Modeling of Mangerak Salt Diapir (South West of Firozabad Fars-Iran)

Mangerak salt diapir is in the South West of Firuz Abad in Fars province, southern Iran and structurally, it is exposed in the simple folded belt of Kohzad Zagros. This diapir, now, is located in a transtentional zone in the overlapping parts of Koreh Bas right fault zone. The origin of this diapir is evaporative series of Hormuz, at the beginning of the Cambrian and ending Precambrian age. In this paper, we investigated tectonic structures around the salt dome of Mangerak. By promoting this salt dome, a variety of structures, such as marginal reverse faults, radial normal faults, folds and caves have been developed. In addition, many changes can be seen in the status line, and the amount and direction of the strata dip, thickness of rock units, and facies change, that all showed the downbuilding phenomenon in the diapir. The method was based on structural desert surveys and relevant measurements. The results show that salt domes above were rising during the Late Cretaceous-Paleocene, and pre-deformation of Zagros and in connection with basement Fault of Korebas, and probably, when sedimentation was in the Zagros basin, they have been exposed in the form of an island. Zagros deformation at the same time, the pressure released from the collision zone on the north side of the East and its wave motion, to the South West, exerts more pressure on the salt horizons and helps them to erupt. About Mangerak salt dome, which is exposed on the side of the Sayakh anticline axis and Basement fault of Korebas, two phenomena are effective in its exposing. Analytical modeling shows the life of the outcrop 31,000 years that this age is consistent with the effects of salt dome Neotectonic.

EARLY NEOPROTEROZOIC CRUST FORMATON IN THE DZABKHAN MICROCONTINENT, CENTRAL ASIAN OROGENIC BELT

The Dzabkhan microcontinent was defined by [Mossakovsky et al., 1994] as a cratonic terrane with an early Precambrian basement that combines highgrade metamorphic complexes of the Songino, Dzabkhan, Otgon, Baidarik, Ider and Jargalant Blocks. However, early Precambrian ages have so far only been recognized in the Baidarik and Ider blocks [Kozakov et al., 2007, 2011; Kroner et al., 2015].

Kimberlite discoveries in NW Liberia: Tropical exploration & preliminary results

This report is brief in context and rich in unexpected discovery. With >2km of erosion, kimberlite models predict the near-complete removal of pipes with exposures to the pipe-root-zones of dikes. Exploration in NW Liberia has, indeed, uncovered eight kimberlite dikes (~10m wide) but also an en echelon pipe, comparable in size to the Kimberley pipe and De Beers' pipe in South Africa. Discoveries are in a narrow 200–300m wide valley of extraordinary thick bush, undergrowth, and organic overburden. Ilmenite and co-existing leucoxene were used as diagnostic tracers for detecting hard rock kimberlite in this tropical terrane. Micro-diamonds show that the redox state of ilmenite is a potentially useful proxy as an index for macro-diamond preservation. The tectonic control of kimberlites is complex, with diverse lithologies. Discoveries include a well-defined regional trend for kimberlite dikes along paleo-fracture zones, Precambrian in age (Liberia Trend), coupled with kimberlite dikes on the craton that are traced to Mesozoic oceanic transform faults (the Sierra Leone Trend). Although long predicted, this is the first report of kimberlite dike-trends in Liberia that are similar in orientation to those in Sierra Leone. An explosive blow on a Liberia-Trend dike demonstrates a similarity to the dynamics attendant in rich (50–500cpht) diamond-bearing dikes in Sierra Leone, and in South Africa of comparable age. The potentially high grade dikes, along with the pipe (~500×50m), now more reasonably accounts for the enormous number of alluvial diamonds (blood and non-conflict), recovered over more than seven decades, downstream from the discovery cluster.A neglected region since the classic work by Bardet (1974), and with few contributions on Liberia since then, an update is considered timely, particularly in the context of discoveries of diamond-bearing kimberlite.

What generated the Late Permian to Triassic unconformities in the southern Junggar Basin and western Turpan Basin; tectonic uplift, or increasing aridity?

The Upper Permian to Triassic sedimentary successions of the southern Junggar Basin and western Turpan Basin are characterized by widespread unconformities. However, the origin of these unconformities remains highly controversial. Three sections, namely the Jimsar section in the southern Junggar Basin and the Aiweiergou and Yuergou sections in the western Turpan Basin, have been systematically studied using detrital zircon U-Pb geochronology, heavy mineral assemblages, and whole-rock geochemical analyses. Detrital zircon ages show that both Upper Permian and Triassic samples from each section have similar age distributions, with a major age group at 250–380Ma, a subordinate age group at 380–540Ma, and sporadic Precambrian ages, suggesting that the main provenance areas were the North Tian Shan and the northern margin of the Yili-Central Tian Shan, with a secondary source area in the Yili-Central Tian Shan. REE distributions and trace element crossplots also indicate that Upper Permian and Triassic samples display similar characteristics in both basins; all were sourced from felsic or mixed mafic/felsic rocks, with only a minor component of ancient or recycled sediments. Detrital provenance and geochemical analyses agree in showing no significant changes in the source-to-sink sediment transport system from the Late Permian to the Triassic. However, field observations, heavy minerals analysis, and geochemistry do suggest that both the southern Junggar Basin and western Turpan Basin experienced an arid climate during this interval. Both Upper Permian and Triassic sediments in the two basins contain abundant hematite, display low CIA values and Th/U ratios, and have mudcracks, redbeds, and other indicators of arid climate. Therefore, we suggest that the regional unconformities in the two basins were not generated by regional tectonic uplift, but most likely resulted from an increasingly arid climate.

Application of electrical resistivity method for groundwater exploration at the Moghra area, Western Desert, Egypt

Moghra territory is one of the most encouraging regions for watered farming and development in the northern part of Western Desert. To achieve that development groundwater is considered the most favorable tool. Geophysical survey in terms of geoelectrical resistivity sounding has been conducted using the Schlumberger array – eleven vertical electrical soundings (VES’s) were measured on the study area. The aim of this study was to investigate the hydrogeological condition at the area under investigation.Three geoelectric sections are prepared; each of them is composed of four units. The first layer has resistivity ranging from 58 to 2029ohm.m and thicknesses from 0.46 to 10.7m characterizing a gravely sand facies of Quaternary age (Pleistocene to Holocene). The second geoelectrical layer has resistivity values ranging between 188 and 518ohm.m which are comparatively high, with various depths varying between 14.7 and 25.8m from Lower Miocene age. It is formed of sand and gravel. The third geoelectrical layer is considered the aquifer for the area under investigation with moderate electrical resistivity values (17–111ohm.m), and could be recognized sometimes as the most extreme depth of penetration. At the maximum depth of penetration of this area, the lower layer (fourth layer) was recognized with the highest electrical resistivity values (1227–8935ohm.m) of basaltic sheet (Precambrian age).Structurally, the study area is influenced by three faults two of them are geological/geoelectrical faults forming a graben structure at the central part; the third fault is a geoelectrical fault and is located to the west of the graben structure forming a horest structure.

Detrital zircon provenance constraints on the initial uplift and denudation of the Chinese western Tianshan after the assembly of the southwestern Central Asian Orogenic Belt

U–Pb and Lu–Hf isotopic data of detrital zircons from late Paleozoic and Mesozoic strata along the southern flank of the Chinese western Tianshan enable to identify provenance changes and reconstruct early stage uplift and denudation history of the Tianshan range. Detrital zircons from Permian and Early–Middle Triassic siliciclastic rocks show two prominent age populations at 500–390Ma and 310–260Ma, and subordinate Precambrian ages at ~2.5Ga, 2.0–1.7Ga, 1.2–0.9Ga and 900–600Ma, with rare ages between 390 and 310Ma. These characteristics and zircon εHf(t) data consistently suggest a sediment source predominantly from the Tarim Craton, rather than the Central Tianshan–Yili Block. In contrast, Late Triassic to Cretaceous strata additionally contain abundant 390–310Ma and 260–220Ma detrital zircons, implying multiple source regions from the Central Tianshan–Yili Block, Tarim Craton, and Western Kunlun Orogen. A significant switch of sedimentary provenances occurred in the mid-Triassic and is consistent with contemporaneous change of paleocurrent directions and the onset of intense tectonothermal events in the broad region of the Chinese western Tianshan and Kyrgyz Tianshan. These data collectively indicate that the significant surface uplift and denudation of the Tianshan range were probably initiated in the mid-Triassic (~240Ma) after the assembly of the southwestern Central Asian Orogenic Belt. This uplifting event represents an intracontinental orogeny most likely in response to the collision between the Qiangtang Block and southern Eurasia, following the closure of the western part of the Paleo-Tethys Ocean.

Evaluate the geological structure, petroleum potential by interpretation the 2D seismic data of Phu Quoc basin

So far, the understanding of Pre-Tertiary formations in the Phu Quoc basin is eliminated due to lack of drilling and the absence of outcrops except on Phu Quoc Island and some other isles. By analogue with stratigraphic information from surrounding areas, seismic horizons may be correlated with known-age strata. Strata of pre-Cambrian and Early Paleozoic ages have not been discovered yet in SW Viet Nam but by analogue may consist of metamorphosed rocks such as chloritized phylite, sericite schist, and quartzite sandwiched with ultra basic rocks. Therefore, PetroVietnam assigned PVEP to perform acquisition of 2D seismic on the area of blocks 41- 44 in order to get additional data for more precise evaluation of the petroleum potential and defining the exploration strategy in the Phu Quoc area.Based on seismic data interpretation in block 42 of Phu Quoc Basin, the Isodepth maps were constructed by approach of conversingtime to depth from isochrone maps. The results of interpretation allowed to identify many seismic closures particularly on the isodepth map of the Brown section, but these closures are located deeply above 5000m, which reliability is fair and economic effect is weak. Hence, the Brown horizon did not considered as structures identified on and concentrate to describe those determined in the Orange and Purple horizons. Result of interpretation indicated 8 closures, which could be identified in the Orange and Purple horizons

Subsurface structure and stratigraphy of the northwest end of the Turkana Basin, Northern Kenya Rift, as revealed by magnetotellurics and gravity joint inversion

In order to understand the subsurface stratigraphy and structure of the northwest end of the Turkana Basin, Northern Kenya Rift, we used 2-D joint inversion of magnetotelluric (MT) and gravity data acquired along 3 profiles perpendicular to the main Murua Rith-Lapur Rift Border Fault. The regional geology is characterized by a basement of Precambrian age overlain by a ≤500-m thick sandstone formation named the Lapur Sandstone of upper Cretaceous-lower Eocene in age, covered by thick rhyolitic and basaltic lavas of late Eocene-middle Miocene age, known as the “Turkana Volcanics”. Final interpretation of the resistivity and density models, until 5 km depth, obtained by the joint inversion approach confirms the previous general knowledge about the half-graben geometry of the northern part of the Turkana Basin. The main Murua Rith-Lapur Rift Border Fault is well identified by both gravity and MT. At least, two other important secondary faults without surface expression are also identified. A new small half-graben basin, named the Kachoda Basin, parallel to the main Turkana Basin and filled by 1.5 km of sediments, has been also characterized. This study also highlights strong thickness variations of the three main geological units that could be expected in the subsurface of the Turkana Basin. For example, the sedimentary Nachukui and Kibish Formations reach up to >3 km in thickness at the eastern end of the north and central profiles. Lateral variations of the topography of the Precambrian basement are also evidenced. Conceptual geological models, which result from the combination of the obtained density and resistivity models as well as from geological and reflection seismic data, are proposed. In such an area of intensive and promising oil exploration, these models are essential in terms of identification of reservoirs, source rocks and trapping mechanisms.

ESTIMATION OF HYDRAULIC PROPERTIES FROM PUMPING TESTS DATA OF NAIROBI AREA, KENYA

The aim of this study was to determine the hydraulic properties of Nairobi area in order to highlight the groundwater potential and to identify the distribution of hydraulic characteristics of aquifers in the area as well as to highlight vulnerability of the aquifer to heavy abstraction. Documented pumping tests data of boreholes located in Nairobi County were analyzed. Pumping test data from eighty four (84) single-well boreholes were analyzed in AQTESOLV software to determine transmissivity and storativity using Cooper-Jacob’s, Theis’s, Papadopolus-Cooper’s and Theis’s recovery methods. Hydraulic conductivity was calculated based on the relationship between hydraulic conductivity and transmissivity. The values of transmissivity for all wells ranged between 1.11 and 360.58 m2/d by using Theis -and Cooper-Jacob methods, 1.10 and 360.58 m2/d by Papadopulos-Cooper method and between 1.289 and 677.81 m2/d by Theis recovery method. The city of Nairobi faces increasing ground-water demand due to ever rising population which is mainly driven by rural-urban migration and industrial growth. Surface water has not only been over-stretched but also heavily polluted and unreliable thus groundwater is the only reliable alternative source of water in the area. Lava and pyroclastic formed during Cenozoic age make up the geology. Under these lavas and pyroclastic rocks lie schists of Precambrian age and gneisses of Mozambique belt of the same age. These results indicate that the aquifer is heterogeneous and that the groundwater supply for local water supply (small communities and plants) is reliable but withdrawal for great regional supply is limited.

The zircon U–Pb and Hf isotope constraints on the basement nature and Paleozoic evolution in northern margin of Yili Block, NW China

The northern margin of Yili Block in Xinjiang, China, consists of Precambrian basement, Early Paleozoic sedimentary rocks intersected with bimodal volcanic rocks, conglomerate and Late Paleozoic volcanic and sedimentary rocks. Five magmatic events are identified: ~2.5Ga, 1.5–1.4Ga, 1.1 to 0.9Ga, 480 to 450Ma and 410 to 350Ma based on SHRIMP zircon U–Pb ages of volcanic rocks and conglomerate in Tulasu Basin in west part of Northern Yili Block margin. The Precambrian ages record the evolution of the basement under Yili Block. The age population of 480 to 450Ma corresponds to the Early Paleozoic bimodal volcanic rock, that of 410 to 350Ma is documented by Late Paleozoic volcanic rocks.The εHf(t) value of ~2.5Ga zircon is 1.0, corresponding to TDM2 of 2.9Ga, indicating the old crust of Yili Block formed during 3.0 to 2.5Ga, which is also evidenced by TDM2 (3.2 to 2.6Ga) of 1.5–1.4Ga aged zircons. The old crust reworked at 1.5–1.4Ga, proved by εHf(t) (−16.3 to −1.3) value of the zircons. The 1.1 to 0.9Ga zircons show εHf(t) value of −7.6 to 2.7 and TDM2 of 2.3 to 1.7Ga, might be the remelting products of old crust with some contribution of mantle. The Precambrian age groups of Yili Block are coeval with Chinese Central Tianshan, Tarim Craton and Kyrgyz North Tianshan, suggesting that they may represent fragments of a single continent characterized by magmatic events at 1.5 to 1.4Ga and 1.1 to 0.9Ga.Bimodal volcanic rocks erupted at 480 to 450Ma, thus a rift-related tectonic setting was proposed in combination with reported geochemical characteristic. The Basaltic rocks with age of ~477Ma have εHf(t) value of 3.4 to 13.9, showing a magma source of depleted mantle with involvement of little crustal material. The felsic rocks with age of ~450Ma have εHf(t) value of −20.4 to 1.2, indicative of a continental crust source and little contribution of mantle magma. Their magma sources also show rift-related magma property. The volcanic rocks erupted during 410 to 350Ma were the products of subduction of the North Tianshan oceanic crust, which made contribution to crustal growth of northern Yili Block. The magma source of the Paleozoic volcanic rocks is depleted mantle contaminated by crust, constraint by their positive εHf(t) value (−0.2~12.8).

Chemical Analysis of the Dust on a Historically Important Collection

The Department of Earth Sciences, Natural History Museum, London, houses an important historical collection: the W. B. Carpenter collection of Eozoon, which is of great significance in the history of science, specifically with regard to a nineteenth-century controversy about the presence of fossils in rock of Precambrian age. After providing a history of the Eozoon Collection, we then give an overview of its care and curation up to the present before describing new research on the history of the specimens and the origin of the thick black dust which covers them. Chemical and morphological analysis of the dust using SEM-EDX (scanning electron microscopy with energy dispersive X-ray spectroscopy) allows identification of one silicate and one sulphur-calcium phase. The silicate may derive from the Eozoon specimens themselves, whereas the sulphur-calcium phase may indicate gypsum from plaster. It is possible that the gypsum dust may have come from collapse of the ceiling during the fire in November 1885 at Carpenter's home that led to his death.

Zircon geochemistry records the action of metamorphic fluid on the formation of ultrahigh-pressure jadeite quartzite in the Dabie orogen

A combined study of mineral inclusions, U–Pb ages, trace elements and Hf-O isotopes was carried out for zircons from a coesite-bearing jadeite quartzite in the Dabie orogen. The results provide insights into the action of ultrahigh-pressure (UHP) metamorphic fluids during continental deep subduction to a mantle depth and thus constraints on the origin of the jadeite quartzite in the continental subduction zone. The zircons mostly show core-rim structures in cathodoluminescence images. The overgrown rims contain rare mineral inclusions, and exhibit concordant U–Pb ages of 225 to 246Ma and flat HREE patterns with negligible Eu anomalies. In contrast, the relict cores contain UHP metamorphic mineral inclusions such as coesite, jadeite and rutile, and show discordant U–Pb ages ranging from 983 to 2045Ma and steep REE patterns with significant negative Eu anomalies. The U–Pb isotope data for the all cores and rims define an apparent discordia line with upper and lower intercept ages of 2000±43Ma and 234±18Ma, respectively. We interpret the rims with Triassic ages as the new growth of metamorphic origin and the cores with Precambrian ages as the protolith relics of magmatic origin. The relict magmatic zircons underwent two subtypes of metamorphic recrystallization, i.e., solid-state transformation and metasomatic alteration. The solid-state recrystallized zircons exhibit slightly discordant U–Pb ages close to the protolith age, steep MREE–HREE patterns, and almost unchanged Hf isotope ratios. These observations point to the lowest degree of resetting to the geochemical composition of protolith zircons. In contrast, the metasomatically recrystallized zircons exhibit partial resetting in protolith zircon REE composition and U–Pb and Lu–Hf isotopic systems. All of the zircon domains, regardless of the rims and cores, show relatively consistent δ18O values of 4.0±0.2‰. Such a consistency indicates not only that the metamorphic fluids are of internal origin from the deeply subducted continental crust but also that the oxygen isotope composition of protolith zircons was reequilibrated with the UHP metamorphic fluids of Triassic age. The metasomatic recrystallization of protolith zircons is indicated by the occurrence of UHP inclusion minerals such as coesite, rutile and jadeite in sealed microcracks. In this regard, the fluid metasomatism firstly took place along fractures of the relict zircons during prograde subduction of the continental crust and then experienced the metamorphic recrystallization to result in sealing of the fractures under the UHP conditions. As such, the metasomatic recrystallization has heterogeneously reset the U–Pb and Lu–Hf isotope systems of protolith zircons. The composition of inclusion minerals within the relict zircon cores suggests that the metamorphic fluids were rich in Si, Ti, Na and Al. These elements would be acquired by the metamorphic fluids through metasomatic reaction of metagreywackes overlying the granitic orthogneiss. Therefore, the jadeite quartzite would be precipitated from the UHP metamorphic fluids that were derived from dehydration of the underlying basement orthogneiss but reacted with the metagreywackes during the continental subduction-zone metamorphism.

Phase change in subducted lithosphere, impulse, and quantizing Earth surface deformations

Abstract. The new paradigm of plate tectonics began in 1960 with Harry H. Hess's 1960 realization that new ocean floor was being created today and is not everywhere of Precambrian age as previously thought. In the following decades an unprecedented coming together of bathymetric, topographic, magnetic, gravity, seismicity, seismic profiling data occurred, all supporting and building upon the concept of plate tectonics. Most investigators accepted the premise that there was no net torque amongst the plates. Bowin (2010) demonstrated that plates accelerated and decelerated at rates 10−8 times smaller than plate velocities, and that globally angular momentum is conserved by plate tectonic motions, but few appeared to note its existence. Here we first summarize how we separate where different mass sources may lie within the Earth and how we can estimate their mass. The Earth's greatest mass anomalies arise from topography of the boundary between the metallic nickel–iron core and the silicate mantle that dominate the Earth's spherical harmonic degree 2 and 3 potential field coefficients, and overwhelm all other internal mass anomalies. The mass anomalies due to phase changes in olivine and pyroxene in subducted lithosphere are hidden within the spherical harmonic degree 4–10 packet, and are an order of magnitude smaller than those from the core–mantle boundary. Then we explore the geometry of the Emperor and Hawaiian seamount chains and the 60° bend between them that aids in documenting the slow acceleration during both the Pacific Plate's northward motion that formed the Emperor seamount chain and its westward motion that formed the Hawaiian seamount chain, but it decelerated at the time of the bend (46 Myr). Although the 60° change in direction of the Pacific Plate at of the bend, there appears to have been nary a pause in a passive spreading history for the North Atlantic Plate, for example. This, too, supports phase change being the single driver for plate tectonics and conservation of angular momentum. Since mountain building we now know results from changes in momentum, we have calculated an experimental deformation index value (1–1000) based on a world topographic grid at 5 arcmin spacing and displayed those results for viewing.

Feldspar dissolution-enhanced porosity in Paleoproterozoic shale reservoir facies from the Barney Creek Formation (McArthur Basin, Australia)

The Paleoproterozoic Barney Creek Formation (BCF; McArthur Basin, Australia) is one of the oldest active hydrocarbon systems on Earth with oil and gas shows present within organic-rich intervals (up to 7 wt. % total organic carbon). We combine bulk geochemical analyses, pore-space characterization, and high-resolution electron imaging techniques to characterize the evolution of the BCF pore system with maturity. A thermal gradient from the pre-oil window (0.48% calc. ) to gas window (1.01% calc. ) shows a progressive change in pore networks with the loss of organic-hosted pores dominant in thermally immature samples to porosity increasingly associated with the mineral matrix with thermal maturity. Precipitation of fine-grained, high surface area cements reduced porosity within the oil window, whereas feldspar and dolomite dissolution and creation of secondary pores increased porosity within the gas window. The abundance of feldspar grains (up to 50%) provided a significant potential for secondary pore formation as well as a source of silica and clay cement. This study identifies a first-order linkage between the chemically reactive sediments that are key to the reservoir properties in the BCF and a provenance and/or weathering intensity conducive to supplying fine-grained, mineralogically immature sediments during deposition. These findings likely apply more broadly to other chemically immature mudrocks typical of Precambrian age sediments or Phanerozoic settings subject to limited chemical weathering.

The Chapada Cu–Au deposit, Mara Rosa magmatic arc, Central Brazil: Constraints on the metallogenesis of a Neoproterozoic large porphyry-type deposit

The large Chapada Cu–Au deposit is situated in Goiás Magmatic Arc, Central Brazil, which is part of an accretionary orogen with tectonic evolution between ca. 900 and 600Ma during the Brasiliano Orogeny. With a current reserve (proven and probable) of 420.94 Mt Cu (0.29%) and 479.84 Mt Au (0.25g/t) or 3.83 Moz Au, the Chapada Cu–Au deposit is an important example of a large-tonnage, low-grade porphyry copper–gold deposit of Precambrian age, in which the mineralization is disseminated and hosted mainly by biotite-rich schists and gneisses.In this work, we propose a two-stage genetic model for the Chapada deposit that includes an early porphyry-type Cu–Au event overprinted by a shear zone-hosted Cu–Au event. The first mineralization event represents a rare case of metamorphosed Neoproterozoic porphyry-type deposit which preserves distinct hydrothermal alteration–mineralization zoning developed at the interface between equigranular (867±8Ma) to porphyritic (884±15Ma) medium- to high-Kcalc-alkaline dioritic rocks and mafic to intermediate metavolcanic and metapyroclastic rocks. The εNd values are positive (from +2.17 to +5.39), thus indicating the juvenile nature of the original rocks for both metavolcano-sedimentary and plutonic rocks. In this context, kyanite-rich rocks are interpreted as products of metamorphosed argillic hydrothermal alteration halo and occur as linear alignments oriented parallel to the Mara Rosa magmatic arc trend. The potassic hydrothermal alteration halo, which includes the main host rocks of the Chapada deposit, is characterized by biotite schist, muscovite–biotite schist, amphibole–biotite schist and biotite gneiss. The ore is represented predominantly by platy deformed chalcopyrite and pyrite disseminations along foliation plans (or axial surfaces of folds) and, to a lesser extent, by small massive concentrations in fold hinges. The ore shows a systematic oxide/sulfide zonation, including the peripheral pyritic shell and the central magnetite-rich zone. In general, the geological context of the deposit lacks preserved stocks, dykes and breccias, while deformed A-type quartz veins/veinlets are abundant.The second mineralization event represents an epigenetic shear zone-hosted Cu–Au mineralization associated with Dn+1 and occurred between 630 and 580Ma during the late orogenic to post-orogenic stage of the Mara Rosa magmatic arc, which may be interpreted as i)a shear zone-hosted Cu–Au remobilization; ii) an orogenic Cu–Au mineralization, or iii) an intrusion-related Cu–Au mineralization. In the latter case, the mineralization could be related to the ilmenite-bearing younger metadiorite (634.8±6.8Ma) with disseminations of chalcopyrite, pyrite and pyrrhotite.The narrow range of pyrite and chalcopyrite δ34S values close to 0‰ (between −2.90 and +0.76‰) indicates that both the porphyry copper–gold mineralization and the epigenetic mineralization were generated by sulfur of magmatic origin.

The stones of the statuary of the Egyptian Museum of Torino (Italy): geologic and petrographic characterization

A geologic and petrographic study was performed on a rich collection of statues made of stone exposed at the statuary of the Egyptian Museum of Turin (NW Italy) to enhance the value of this artistic heritage and set the basis for its best conservation. Magmatic and sedimentary rocks were recognized. Magmatic rocks with an intrusive origin are the most represented and include two main varieties: Red Granite, consisting of a sieno-granite with porphyritic texture and pink to red K-feldspars, and Black Granite, which includes granodiorite, quartz diorite and tonalite lithotypes, whose colour ranges from grey to almost black. These magmatic rocks belong to the Arabian–Nubian shield, and the historical quarries are located near Aswan. The sedimentary rocks are represented by Cenozoic white limestones and red sandstones and Cretaceous dark-yellow Nubian sandstones. Finally, we note the occurrence of the so-called Bekhen Stone, originally attributed to a green-black metagreywacke belonging to the Hammamat series of late Precambrian age, outcropping in the central sector of the Eastern Desert, and re-interpreted here as a massive dark-green sandstone. This paper provides a scientific classification of the artefacts exposed in the statuary rooms based on the employed materials and contributes to the enhancement of the valuable collection of stone artefacts preserved in one of the leading ancient Egyptian Museums in the world.