New in situ Lu–Hf data on zircons from GSWA geochronology samples has provided a unique isotopic dataset with a high temporal resolution for the Murchison Domain of the Yilgarn Craton in Western ...Australia. These data identify extended periods of juvenile mantle input (positive εHf values) into the crust firstly at c. 2980Ma and then from c. 2820Ma to c. 2640Ma with significant pulses of crustal recycling at c. 2750Ma and c. 2620Ma (highly negative εHf values). Geochemical data from well-characterised granitic suites of the Murchison Domain provide additional constraints on the crustal evolution of the area and indicate a prolonged period of crustal melting and remelting at progressively shallower depths from c. 2750 to c. 2600Ma.
At c. 2760–2753Ma, widespread calc-alkaline, intermediate to silicic volcanic rocks of the Polelle Group were erupted, accompanied by intrusion of felsic to intermediate melts derived from a variety of crustal sources that likely formed by partial mixing with basaltic melts. The intrusive rocks include a wide geochemical array of rocks in the Cullculli and Eelya suites that were sourced over a wide range of crustal depths. At this time a major departure to negative εHf values (<−5) occurred, indicating sampling of c. 3.80Ga model aged source rocks as well as continued juvenile input. Post-volcanic granitic rocks emplaced between c. 2710 and c. 2600Ma show geochemical evidence for progressive fractionation through time and derivation from an evolving crustal source.
We interpret the driving force for this protracted history of mantle and crustal melting to be two mantle plumes at 2.81 and 2.72Ga. These data document the process of cratonization through progressive melt depletion of the lower crust, progressively fractionating and shallower melts, culminating with a final phase of crustal recycling (εHf<−5) and the cessation of juvenile input at c. 2630–2600Ma during intrusion of the Bald Rock Supersuite, resulting in cratonization of this part of the Yilgarn Craton.
► Murchison Hf data are consistent with plume activity at 2.81 and 2.72Ga. ► A significant juvenile event is identified at 3.04Ga. ► Polelle Group is unique in that indicates significant reworking of older crust. ► Reworking is accompanied by diverse melt sources. ► Granites become shallower sourced and more fractionated, leading to cratonization.
A palynological study of 239 outcrop samples and their sedimentological context was undertaken on the Pliocene Productive Series in the Kirmaky and Yasamal valleys, eastern Azerbaijan. The Productive ...Series is primarily a representation of the palaeo-Volga and forms the main hydrocarbon-producing reservoirs in the South Caspian Basin. Most sands are interpreted as fluvial, based on sedimentary characteristics. Mudstone and siltstones often contain freshwater and brackish assemblages interpreted as 'Caspian lake' transgressions, indicative of rapid Caspian Sea level change during the Pliocene. Most samples contain rich assemblages including pollen, spores, dinoflagellate cysts, algae and fungal bodies. Common tree pollen elements include Pinus, Alnus, Betula, Carya, Juglans, Pterocarya, Quercus and Ulmus, which all occur in present-day vegetation or pollen records from the Caucasus or Urals. Herbaceous pollen includes Amaranthaceae, Asteraceae (including Artemisia), Ephedra and Poaceae, commonly found in the drier Caspian regions. The dinoflagellate cysts include 'Peri-paratethyan endemic' taxa such as Caspidinium rugosum and Spiniferites cruciformis. 'Pannonian' species such as Thalassiphora balcanica, Romanodinium areolatum and Spiniferites oblongus confirm the persistence of these taxa into the early Pliocene in Eastern Paratethys, around five million years later than their first documented presence in Central Paratethys. 'Caspian lake' influences diminish up-section, as indicated by a progression from brackish to freshwater and sub-aerial conditions. Productive Series deposition was mainly driven by the combined effects of lake level and catchment climate. The principal hydrocarbon reservoir sands were deposited as a result of increased catchment humidity, whereas drying conditions led to reduced coarse clastic input and deposition of alluvial plain mudstones. Productive Series deposition terminated with the onset of the marine-influenced Akchagyl Series, which spans the Plio-Pleistocene boundary. The lowermost sediments of the Akchagyl Series are freshwater in origin and grade up-section into marine beds containing dinoflagellate cysts of Arctic affinity.
The Tapajós Mineral Province (TMP) contains evidence of subduction processes during the Paleoproterozoic. Despite well-documented arc magmatism, the lack of solid geological evidence indicating a ...collision raises uncertainties about whether there was an arc-continent collision in the Paleoproterozoic or the process of subduction continued, and the closing of an ocean basin occurred only in Mesoproterozoic times. Additionally, world-class deposits have been linked with lithospheric electrical conductors using magnetotelluric (MT) data. On the lithospheric scale, an assessment of the potential for more significant deposits or correlated commodities is absent in the TMP. Therefore, we acquired MT data to provide constraints for the lithosphere of the TMP. The electrical resistivity model reveals a high resistivity zone (> 10,000 Ω.m) interpreted as the emplacement zone of granitoids in the upper crust. The mid-lower crust presents resistivity between 465 and 45 Ω.m, corresponding to depths of 10–35 km. A low resistivity zone below 35 km (45–10 Ω.m) represents a hydrated upper mantle due to the long-lived arc processes. Based on gravity-forward modeling, we estimate that the degree of serpentinization is <10% due to post-subduction magmatism. The lack of a lower crust conductor and paired high and low long wavelength Bouguer anomalies corroborates that a cryptic suture is unlikely. A low-angle conductor matches the limit of different magnetic domains and reveals the exhumation of the western portion of TMP's crust. The high correlation between the position of the deposits and electrical conductors corroborates that MT is a valuable tool for detecting fossil fluid pathways relevant to finding new mineral deposits.
•The Tapajós Mineral Province presents an upper mantle of low resistivity due to metasomatization processes.•We estimate that the degree of hydration is less than 10% based on gravity data.•We mapped crustal fluid pathways with a high correlation with gold deposits.
A major outstanding question concerning the Bushveld Complex of South Africa is the genetic relationship between the granites (Lebowa Granite Suite, LGS) and granophyres (Rashoop Granophyre Suite, ...RGS), and their relation to the underlying layered mafic-ultramafic intrusion (Rustenburg Layered Suite, RLS). Here, we present new bulk rock major and trace elements, as well as Nd and Hf isotopes for 35 samples from the LGS (Nebo Granite) and RGS (Stavoren Granophyre and Zwartbank Pseudogranophyre) and combine this with published data to form a comprehensive dataset. The LGS and RGS range from ferroan to peraluminous compositions. Relative to the bulk continental crust, the granite and granophyres are depleted in Sc, V, Ti, P, Cr, and Sr, and either similar or enriched in other trace elements. εHf and εNd values (at 2055 Ma) show narrow ranges between −9.1 ± 0.8 to −6.8 ± 0.7 and − 6.0 ± 0.3 to −5.1 ± 0.6, respectively, across hundreds of kilometres, although higher εNd values have been reported previously (up to +5.2). These isotopic compositions correspond to published values for the RLS. Thus, the major element and isotopic composition as well as general trace element trends observed for LGS and RGS are consistent with derivation through fractional crystallization of basaltic magmas. A Rayleigh fractionation model for trace elements and major element mass balance suggest that c. 20% granite can be produced from fractional crystallization of such magmas. Multiple lines of evidence suggest that the granitic magmas mainly evolved in underlying, staging magma chambers rather than in the RLS chamber itself. This includes evidence for magma mixing in a newly identified transition zone separating RLS and LGS, the finding of zircon antecrysts in LGS and RGS, and the replenished and tapped nature of the RLS chamber.
The crystalline basement beneath the Cretaceous to Cenozoic Bight and Eucla Basins, in Western Australia has received comparatively little attention even though it lies on the eastern margin of one ...of the most mineral resource endowed regions on the planet. This basement is characterized by a complex geological evolution spanning c. 2billionyears, but paucity of outcrop and younger basin cover present a daunting challenge to understand the basement geology. In this work the composition of the unexposed Proterozoic crystalline basement to the Bight and Eucla Basins is investigated through zircon Hf isotopes and whole rock geochemistry from new drillcore samples. This region includes two geophysically defined basement entities: The Madura Province, containing: 1) c. 1478Ma Sleeper Camp Formation, which has variable isotopic signatures including evolved values interpreted to reflect reworking of rare slivers of hyperextended Archean crust, 2) 1415–1389Ma Haig Cave Supersuite, with mantle-like isotope values interpreted as melting of subduction-modified N-MORB source, and 3) 1181–1125Ma Moodini Supersuite, with juvenile isotopic signatures interpreted to reflect mixed mafic lower-crustal and asthenospheric melts produced at the base of thinned crust. The Coompana Province, to the east of the Madura Province, has three major magmatic components: 1) c. 1610Ma Toolgana Supersuite, with chemical and isotopic characteristics of primitive arc rock, 2) c. 1490Ma Undawidgi Supersuite, with juvenile isotope values consistent with extensional processes involving asthenospheric input and 3) 1192–1140Ma Moodini Supersuite, with strong isotopic similarity to Moodini Supersuite rocks in the Madura Province.
This new isotopic and geochemical data shows that the Madura and Coompana regions together represent a huge tract of predominantly juvenile material. Magma sources recognised, include; 1) depleted mantle, producing MORB-like crust at c. 1950Ma, but also contributing to younger magmatism; 2) recycled c. 1950Ma crust reworked in primitive arcs and in intra-plate settings and; 3) minor evolved material representing fragments of hyperextended continent. The observed isotopic evolution pattern is comparable to that of other central Australian Proterozoic provinces, including the Musgrave Province, the northern margin of the Gawler Craton, and components within the Rudall Province. Linking these isotopic signatures defines the Mirning Ocean, and its subducted and underplated equivalents. In a global context we suggest c. 1950Ma crust production reflects the onset of ordered oceanic spreading centres, which swept juvenile crustal fragments into Nuna.
•Geochemistry, Nd and zircon Hf isotopes from crystalline Eulca basement.•Results indicate huge tract of predominantly juvenile material.•Major magma source is depleted mantle producing MORB-like crust.•Only minor evolved material that represents fragments of hyperextended continent.•Onset of oceanic spreading at 1950Ma swept juvenile crust into Nuna.
The Late Palaeozoic voluminous magmatism in Transbaikalia, Russia (a territory of >
600,000
km
2 to the east of Lake Baikal) is highly diverse and complex. Of special interest are (1) the significant ...overlap in time between magmatic suites commonly ascribed to post-collisional and within-plate settings and (2) the provenance of the coeval, but distinct, granitoid magmas that are closely spaced within a large region. Magmatic activity lasted almost continuously from ~
330
Ma to ~
275
Ma and included five igneous suites occupying a total area of ~
200,000
km
2: (1) the Barguzin suite of high-K calc-alkaline granite (330–310
Ma); (2 and 3) the coeval Chivyrkui suite of low-silica calc-alkaline granitoids and the Zaza suite of high-K calc-alkaline to alkaline granite and quartz syenite which were emplaced between 305 and 285
Ma; and (4 and 5) the partially overlapped in time Lower-Selenga monzonite–syenite suite (285–278
Ma) and the Early-Kunalei suite of alkali-feldspar and peralkaline quartz syenite and granite (281–275
Ma). The overall increase in alkalinity of the granitoids with time reflects the progress from post-collisional to within-plate settings. However, a ~
20
m.y. long transitional period during which both calc-alkaline and alkaline granitoids were emplaced indicates the coexistence of thickened (batholiths) and thinned (rift) crustal tracts.
Sr–Nd–O isotope and elemental geochemical data suggest that the relative contribution of mantle-derived components to the generation of silicic magmas progressively increased with time. The high-K calc-alkaline granite magmas that formed the Angara–Vitim batholith were generated by high degree melting of supracrustal metamorphic rocks εNd(t)
=
−
5.7 to −
7.7; δ
18
O(Qtz)
=
12‰, with minor contribution of H
2O and K from the underplated mafic magma (the convective diffusion model). The coeval calc-alkaline Chivyrkui suite and the transitional to alkaline Zaza suite formed as a result of mixing of crustal silicic and mantle-derived basic melts in roughly equal proportions. In so doing, the former crystallized immediately from the hybrid magmas, whereas the latter (Zaza suite) formed by fractional crystallization of the hybrid melts following magma mixing. Finally the partly overlapping in time monzonite–syenite (Lower-Selenga) suite and highly alkaline syenite–granite (Early-Kunalei) suite were produced from the similar K-rich basalt source. For the former suite, the magma generation process was dominated by fractional crystallization of K-rich basalt magma. In contrast, the highly alkaline granitoid magmas were likely generated by partial melting (~
20%) of K-rich mafic rocks in the lower crust.
► In Transbaikalia the total territory of 200,000
km
2 is occupied by the diverse Late Palaeozoic granitoids. ► The Late Palaeozoic magmatic cycle lasted about 55
m.y., and the post-collisional and within-plate granite suites overlap in time up to 20
m.y. ► The proportion of mantle-derived components in granitoids progressively changed in time from few percent in early calc-alkaline granite to 100% in peralkaline quartz syenite and granite. ► The coeval and closely spaced, but geochemically distinct igneous suites originated from the same sources.
