Provenance Of Metasedimentary Rocks Research Articles

Provenance of metasedimentary rocks from the Rewat hill, Degana, NW India

Provenance of metasedimentary rocks from the Rewat hill, Degana, NW India

Detrital Zircon Geochronology and Provenance of Metasedimentary Rocks from the Susong Complex Zone in the Dabie Orogen

AbstractThe Susong complex zone is a relatively low‐grade metamorphic unit located in the southern part of the Dabie orogen and preserves a variety of metasedimentary rocks, mostly with epidote–amphibolite facies. However, their depositional age, provenance and tectonic setting of sedimentary protolith remain controversial due to the lack of fossils, precise dating and integrated geochemical investigation. This study has conducted whole‐rock elemental, and zircon U‐Pb SHRIMP dating and Lu‐Hf isotope analyses on three types of representative metasedimentary rocks including garnet‐bearing mica‐quartz schist, graphite‐muscovite‐quartz schist and dolomitic marble from the Susong complex zone. The U‐Pb SHRIMP dating data indicate that the sedimentary protolith of these rocks have the maximum depositional ages of less than 840–750 Ma and have various sedimentary provenances. In combination with zircon Lu‐Hf isotope compositions, the provenance of the metasedimentary rocks in the region is for the first time documented to be mainly derived from four groups of magmatic rocks formed at ∼2.5 Ga, ∼2.0 Ga, ∼1.4 Ga and ∼0.8 Ga in response to four episodes of igneous activity in the northeastern margin of the Yangtze Block. The conventional geothermobarometry combined with rare metamorphic zircon ages indicate that the studied rocks underwent the Late Triassic continental subduction‐related metamorphism with peak epidote–amphibolite facies conditions at P = 0.34–0.91 GPa and T = 427–532°C. In addition, results of the elemental contents (i.e., La, Ce, Th and ΣREE) and ratios (i.e., Eu/Eu* and LaN/YbN) suggest that the protoliths of the metasedimentary rocks were mainly shales, wackes and limestones, most probably related to the Rodinia supercontinent rifting along the northern margin of the Yangtze Block during the Neoproterozoic.

Provenance and tectonic setting of the Neoproterozoic meta-sedimentary rocks at southeastern Tibetan Plateau: Implications for the tectonic affinity of Yidun terrane

The Yidun terrane is located between the Qiangtang terrane and Songpan-Ganzi Fold Belt and its tectonic affinity with other adjacent terrane is of significance to understand the tectonic evolution of southeastern Tibetan Plateau. In this study, we present integrated bulk geochemical data, detrital zircon U-Pb ages and Hf isotopes of meta-sedimentary rocks from the Qiasi Group, which represents the oldest Precambrian basement of the Yidun terrane, to constrain the depositional age and provenance of the meta-sedimentary rocks as well as the tectonic affinity of the terrane. Zircon U-Pb dating results show that detrital zircons from three samples have almost identical age spectra with two major age clusters of 800–780 Ma and 850–820 Ma at age peaks of ca. 790 Ma and ca. 827 Ma, and one minor age cluster of 910–860 Ma. The youngest group of detrital zircons reveals that the maximum depositional age of the Qiasi Group was 779–777 Ma. Low SiO2/Al2O3 (2.9–4.1), CIA (59–47), and PIA (63–47) values indicate that the sediments could be derived mainly from the low mature sources that had undergone weak to mildly moderate degree of chemical weathering. Bulk geochemical and zircon U-Pb-Hf isotopic data suggest that the provenance was composed mainly of Neoproterozoic intermediate to felsic igneous rocks. In combination with the published geological data and detrital zircon U-Pb age spectra on the western Yangtze Block, and the Kunlun-Qilian-Qaidam Block, we propose that the Neoproterozoic basement of the Yidun terrane has a close tectonic affinity with the former one.

Source characteristics and provenance of metasedimentary rocks from the Kangxiwa Group in the Western Kunlun Orogenic Belt, NW China: Implications for tectonic setting and crustal growth

The West Kunlun Orogenic Belt is a result of the closure of Proto– and Paleo–Tethys Ocean and subsequent continental collision. In order to better understand the tectonic evolution of this orogenic belt, whole-rock geochemistry of major and trace elements, detrital zircon U–Pb geochronology and Lu–Hf isotopic composition were determined for metasediments of the Kangxiwa Group (KG) in the South Kunlun Terrane (SKT). These rocks have low Chemical Index of Alteration and high Index of Compositional Variability values, indicating relatively weak chemical weathering and a source compositionally dominated by immature material that lacks alumina-rich minerals. The rocks are all enriched in light rare earth element and display relatively flat heavy rare earth element patterns. Their low contents of ferromagnesian (e.g. Cr, Co and Ni) and high field strength elements (e.g. Zr, Hf, Nb, Ta and Y) are similar to those of graywackes from active continental margins. Detrital zircons from three samples of the KG metasediments can be divided into igneous and metamorphic grains. Zircon U–Pb dating of the igneous grains shows similar age spectra dominated by Early Paleozoic to late Paleozoic ages. The Precambrian igneous grains yield Neoproterozoic 206Pb/238U ages and a few early Mesoproterozoic to Paleoproterozoic 207Pb/206Pb ages. The youngest igneous zircons define the maximum depositional age at ~307Ma for the KG metasediments. The U–Pb dating of all the metamorphic grains yield 206Pb/238U ages at ~245Ma, suggesting the metasediment rocks were deposited prior to the early Triassic. These results indicate that the KG metasediments were deposited on an active continental margin during the time between late Carboniferous and late Permian. Lu–Hf isotopic analyses of the igneous detrital zircons from the three samples show large variations of εHf(t) values mostly between −21 and +12. Combined with their U–Pb ages, two respective crustal growth events are identified in the SKT at the Early Paleozoic and the late Carboniferous.

Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

We present results of study of the trace-element and Lu–Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at ~1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)n ratios, which is typical of igneous zircon. The weak positive correlation between 176Lu/177Hf and 176Hf/177Hf in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in Hf = the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at ~2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of εHf values (from +9.8 to –3.3) and model age TC 2.8–2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope Hf parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive εHf values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower εHf and higher TC values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (εHf values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga.

Maximum sedimentation ages and provenance of metasedimentary rocks from Tinos Island, Cycladic blueschist belt, Greece

U–Pb zircon ages of five metasedimentary rocks from the Lower Unit on Tinos Island (Cycladic blueschist belt, Greece) document supply of detritus from various Proterozoic, Paleozoic and Mesozoic source rocks as well as post-depositional metamorphic zircon formation. Essential features of the studied zircon populations are Late Cretaceous (70–80 Ma) maximum sedimentation ages for the lithostratigraphic succession above the lowermost dolomite marble, significant contributions from Triassic to Neoproterozoic source rocks, minor influx of detritus recording Paleoproterozoic and older provenance (1.9–2.1, 2.4–2.5 and 2.7–2.8 Ga) and a lack or paucity of zircons with Mesoproterozoic ages (1.1–1.8 Ga). In combination with biostratigraphic evidence, the new dataset indicates that Late Cretaceous or younger rocks occur on top of or very close to the basal Triassic metacarbonates, suggesting a gap in the stratigraphic record near the base of the metamorphic succession. The time frame for sediment deposition is bracketed by the youngest detrital zircon ages (70–80 Ma) and metamorphic overgrowths that are related to high-pressure/low-temperature overprinting in the Eocene. This time interval possibly indicates a significant difference to the sedimentation history of the southern Cyclades, where Late Cretaceous detrital zircons have not yet been detected.

Provenance of metasedimentary rocks from the Ceará Central Domain of Borborema Province, NE Brazil: implications for the significance of associated retrograded eclogites

In the Forquilha area (NE Brazil), in NW Borborema Province, high to ultra-high pressure rocks are an important geological key to understanding West Gondwana amalgamation. U–Pb geochronological data for a retrograded eclogite sample yielded an upper intercept age of ca. 1520 Ma and a lower intercept age of ca. 620 Ma. These ages most likely represent the crystallization age of the basaltic protolith and the regional metamorphism, respectively. The retrograded eclogites are enclosed in migmatized quartz-feldspathic gneiss and sillimanite (after kyanite)-garnet-biotite gneiss. Detrital U–Pb zircon data for these paragneisses show only Paleoproterozoic zircon grains with ages clustering from ca. 1800 Ma (the maximum depositional age) to ca. 2480 Ma, and frequency peaks at 2.2–2.0 Ga. Combined with Nd isotopic data from the Forquilha paragneisses, one can assume a single Paleoproterozoic source. Basement rocks of the Ceará Central and the Rio Grande do Norte domains are the most likely candidates. The absence of Meso- and Neoproterozoic zircon grains suggest that the retrograded eclogite bodies possibly do not represent slivers of oceanic rocks captured in active margin sequences during subduction. It was identified that the high-pressure rocks of the Forquilha area are in tectonic contact with high-pressure granulite facies rocks of the Ceará Complex (Independência unit) that present detrital zircon records of an active margin setting, with ages ranging from ca. 660 Ma to 2200 Ma. Metamorphism of this sequence occurred at ca. 650 Ma. Considering previous studies, field relationships, and metamorphic paragenesis, a tectonic scenario is inferred, in which the Forquilha retrograded eclogites represent Mesoproterozoic basaltic rocks of an extensional event that were metamorphosed under eclogite facies conditions during Late Neoproterozoic continental subduction/collision, and juxtaposed to an active margin sequence during the exhumation process.

Provenance of metasedimentary rocks from the Beishan orogenic collage, southern Altaids: Constraints from detrital zircon U–Pb and Hf isotopic data

The origin and tectonic settings of metasedimentary sequences in the Central Asian Orogenic Belt have been a matter of debate regarding their contributions with some proposals of being microcontinents or accreted material, largely due to a lack of high resolution geochronological data. This paper reports detrital zircon U–Pb age and Hf isotopic data for the previously mapped Precambrian metasedimentary rocks from the Beishan orogenic collage, southern Altaids. Our data show that Precambrian ages dominate all the analyzed samples, but two samples yield Paleozoic zircons which suggest that they were not deposited in the Precambrian. The late Paleoproterozoic–early Mesoproterozoic group (~2000–1300Ma) is the largest age population among the six samples analyzed. This age population (~2000–1300Ma) corresponds to the assembly and subsequent break-up of the Columbia supercontinent. Only one sample (11SYS01) yields Neoproterozoic ages (with two peaks at 930 and 785Ma), which shows a possible affinity with the Tarim Craton. Hence, the age spectra presented here are generally different from that of the Tarim Craton and the metasedimentary rocks from the Central Tienshan. Our data show that the Tarim Craton is not the main source area for the metasedimentary rocks from the Beishan orogenic collage, but instead multiple source areas may have contributed to the Beishan collage. Combining our new results with published data, we favor an allochthonous origin for the metasedimentary sequences which may be associated with major thrust tectonics. Therefore, a long-lived arc accretionary model is proposed for the tectonic evolution of the Beishan orogenic collage.

Massive granitoid production without massive continental-crust growth in the Chinese Altay: Insight into the source rock of granitoids using integrated zircon U-Pb age, Hf-Nd-Sr isotopes and geochemistry

This study integrates new and extant zircon U-Pb age and Hf isotope data, whole-rock Nd-Sr isotope data, and geochemistry data from the Chinese Altay (CA) and adjacent areas, in an attempt to identify the source rocks of granitoids and acidic-intermediate volcanic rocks and, hence, continental-crust growth in the CA. The protolith of the gneisses includes the earliest products of erosion from the surface rocks of the island arc volcanics and the syn- and post-collisional granitoids of the Caledonian orogenic-mountain system, whereas the provenance of metasedimentary rocks was exposed from increasingly deeper levels or from an expanded region that evolved from the axis to the flank of the mountain system as it was denuded until planation such that eroded material from the Tuva-Mongol microcontinental block to the east could be delivered to the CA. The CA tends to be more mafic and isotopically more primitive with depth, because material eroded early constitutes the lower section and material eroded late constitutes the upper section of the Early-Paleozoic sedimentary-pile. Source rocks for the granitoids and acidic-intermediate volcanic rocks of the CA include the eroded material from the microcontinental block, the eroded material from the syn- and post-collisional granitoids of the Caledonian province, MORB-type mafic rocks, and a minor component from the metasomatized lithospheric-mantle. Eroded material from the terrains north of the CA was first deposited as an Early-Paleozoic sedimentary-pile in the CA. The top section of this pile was again eroded and transported to the trench at the CA continental margin, and these second-round sediments were subducted beneath the CA accretionary wedge. The un-eroded lower section of the Early-Paleozoic sedimentary-pile lay at bottom of the CA accretionary wedge that was underplated by MORB-type mafic rocks. The CA granitoids can be divided into orogenic (460-360 Ma) and post-orogenic (∼320-260 Ma). The orogenic granitoids can be subdivided into Groups 1 (ɛ_Nd (<i>t</i>) &lt; +1) and 2 (ɛ_Nd (<i>t</i>) mostly higher than +1). The main branch of the granitoids (67 data) that includes Group 1 and a majority of Group 2 were derived mainly from eroded materials from the Caledonian province and the Tuva-Mongol microcontinental block. The source rock of acidic-intermediate volcanic rocks was dominantly eroded materials from the latter. The granitoids in the Erqisi nappe at the junction between the CA and the Junggar are characterized by a mixed source between MORB-like mafic rocks and juvenile crustal-rocks. The Mid-Devonian dacite-rhyolite and dolerite dikes in the west Chinese Altay were derived from metasomatized lithospheric-mantle. Integrated isotope and geochemistry data further constrain the source compositions of granitoids. The granitoids in the northwest Chinese Altay were mainly derived from subducted sediment that was eroded from the top section of the Early-Paleozoic sedimentary-pile, whereas those in the east Chinese Altay were derived dominantly from the lower section of the pile. The source rock of the granitoids in the northeast of the middle Chinese Altay consists of the lower section of the Early-Paleozoic sedimentary-pile; and sediment, quartz keratophyre, and keratophyre that were subducted or thrust beneath the northern Chinese Altay. The main source rock of the granitoids in the southwest of the middle Chinese Altay varies between the subducted sediment in the westernmost segment and the lower section of the Early-Paleozoic sedimentary-pile and the mafic underplate to the east. A geophysical-sounding profile across the CA and East Junggar reveals a three-layer structure of the CA accretionary-wedge underplated by an oceanic lithosphere that was detached from the downgoing oceanic-spreading-center. From the Silurian to the Mid-Late Devonian, the active spreading-center was repeatedly subducted beneath the CA along a southwestward-retreating subduction zone as represented successively by four NE-dipping thrust faults that converge to a low-angle décollement at depth. Asthenospheric upwelling heated and melted the subducted sediment and the lower section of the Early-Paleozoic sedimentary-pile, along with the mafic underplate and generated the acidic-intermediate volcanic rocks and granitoids that culminated at ∼400 Ma. During the Mid-Devonian, shallow dipping dècollement-like subduction changed to a steeply dipping subduction beneath the lithospheric-mantle wedge. The mantle wedge shielded the CA crust from heating from the subducting spreading-center, which resulted in waning granitoid magmatism from ∼400 Ma to ∼360 Ma. In conclusion, there was massive granitoid production but not massive continental-crust growth in the CA at ∼400 Ma.

Protolith age and provenance of metasedimentary rocks in Variscan allochthon units: U–Pb SHRIMP zircon data from the Orlica–Śnieżnik Dome, West Sudetes

AbstractNew U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) data from detrital zircons within the Orlica–Śnieżnik Dome provide new insights into the stratigraphic and palaeogeographic position of assumed relict Precambrian basement preserved in the Variscan collisional orogen of the West Sudetes, SW Poland. Hitherto, the Młynowiec Formation and the Stronie Formation within the Orlica–Śnieżnik Dome were assumed to represent two metavolcano-sedimentary successions of Proterozoic and early Palaeozoic age, respectively. However, when previous U–Pb data and mapping data from the Orlica–Śnieżnik Dome are combined with the new detrital zircon isotopic ages both from paragneisses within the Młynowiec Formation and from light-coloured quartzites and mica schists within the Stronie Formation, the result strongly suggests that the protoliths of these two formations actually form a continuous succession. This continuous succession is herein designated the Młynowiec–Stronie Group. The rocks of this group were deposited during middle Cambrian–early Ordovician times (c.520–470 Ma), presumably at the northern edge of West Gondwana after the 10–20 Ma period of tectonic quiescence that followed the terminal stage of the Cadomian collisions. Monotonous Młynowiec metagreywackes form the lower part of the succession, and the lithologically diverse schistose Stronie Formation forms its upper part. The change from greywacke (Młynowiec) to volcano-sedimentary (Stronie) facies coincided with the onset of rather short-lived volcanic activity which climaxed around 505–495 Ma and which supplied the volcanogenic components to the Stronie Formation. No ‘Cadomian unconformity’ has been observed in the region. Xenocrystic zircons from the Młynowiec–Stronie Group retain records of Archaean (3.0–2.3 Ga), Palaeoproterozoic (2.1–1.8 Ga) and Neoproterozoic to early Cambrian (660–530 Ma) zircon-forming events. These zircon ages, together with the lack of 1.7–1.2 Ga zircon ages, suggest that the source areas for the metasedimentary rocks may have been the West Africa craton, which therefore differs from the Amazonian affinity of the adjacent Brunovistulia Terrane. Nevertheless, two zircons,c.1.0 and 1.1 Ga old, respectively, indicate that the Młynowiec–Stronie Group sedimentary basin must have still been within the delivery reach of detritus ultimately derived from the Grenvillian-age belt(s). The detrital components of the supracrustal formations of the Orlica–Śnieżnik Dome were mainly derived from Neoproterozoic zircon-bearing crystalline rocks that were accreted to, and included in, the Cadomian basement in several intrusive pulses that culminated at 660–640 Ma, 620 Ma and 570–530 Ma.

Provenance of metasedimentary rocks in the northern Gawler Craton, Australia: Implications for Palaeoproterozoic reconstructions

Isotopic, geochemical and U–Pb zircon geochronological characteristics of the Nawa Domain in the South Australian Craton, Australia, have been determined to place constraints upon Palaeoproterozoic reconstruction models. U–Pb detrital zircon analysis of the Nawa Domain metasedimentary rocks indicates deposition between ca. 1740 and 1720 Ma. Sedimentary rocks are shown to be enriched in REE and sourced from evolved crust ( ɛ Nd(1.6 Ga) = −7.0 to −4.3). The combination of geochemical, Nd-isotope and U–Pb detrital zircon age data indicate the dominant source for the studied metasedimentary rocks is likely to be the Arunta region of the North Australian Craton. This indicates the Nawa Domain crustal segment was proximal to the North Australian Craton during the period ca. 1740–1720 Ma. Provenance correlations drawn between the Nawa Domain metasedimentary rocks, lower part of the Willyama Supergroup in the Curnamona (Broken Hill) region and Maronan Supergroup in the Mount Isa region suggest contiguity of these crustal terranes during the late Palaeoproterozoic. These spatial and temporal constraints negate some reconstructions of Palaeoproterozoic Australia suggested for this period, and indicate further refinement of other models is needed. Further, the intra-continental nature and Australian sourced detritus of the lower sections of these basins limits their use as piercing points between eastern Australia and possible neighbouring continents in the Palaeo- and Mesoproterozoic.

Geochemistry and provenance of metasedimentary rocks from the Archean Golden Pond sequence (Casa Berardi mining district, Abitibi subprovince)

The Archean Golden Pond sequence is made up of deformed and metamorphosed conglomerates, greywackes, and mafic volcanic rocks, and is overlain by ferrugineous metasedimentary rocks of the North iron formation. The clastic rocks were derived mainly from a volcanic source that had undergone weak chemical weathering. Their source area was dominated by the presence of 60–80% high-Al2O3 felsic volcanics having strongly fractionated [La/Sm]N (= 3.7 ± 0.3) and very low Ta/Th ratios (= 0.09 ± 0.02), with lesser proportions of basaltic (10–30%) and ultramafic volcanic rocks (1–10%). The ferrugineous metasedimentary rocks can be modelled by mixing 20–40% siliciclastic material, of the composition of the average Golden Pond greywacke, with an Fe- and Si-rich precipitate (molecular Fe/Si = 0.6 ± 0.2). The high-Al2O3 felsic source rocks were most likely produced by subduction processes within an oceanic arc environment, but the mafic and ultramafic volcanic rocks were derived by different processes from an asthenospheric mantle source, possibly in an oceanic rift environment. Therefore, it is suggested that the ultramafic, mafic, and felsic volcanic rocks were brought to the same erosional level by dissection of the arc system and rapid exhumation of the felsic arc lithologies and the deeper ocean floor. Intrabasinal hydrothermal activity associated with contemporaneous mafic volcanism and (or) graben development may have also been responsible for the local production of the Fe-rich precipitates of the North iron formation.

The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses: implications for Archean sedimentation and tectonics in the Superior Province

U Pb isotopic analyses have been performed on single detrital zircon grains from greywacke samples collected along the northern boundary of the Quetico Subprovince in northwest Ontario. Secondary lead loss was virtually eliminated by choosing crack-free grains and removing the outer surface using air abrasion. Thus, the 207Pb 206Pb age can be considered a good approximation to the age of crystallization of each grain. Zircons in the Quetico metasediments show a range of ages from 2698 ± 3 Ma to 3009 ± 4 Ma. Deposition and metamorphism of the metasediments is bracketed within a 10 ± 5 Ma time span between crystallization of the youngest detrital zircon grain and a late-tectonic intrusion, the Blalock pluton, which cuts the sediments and has a zircon age of 2688 ± 4 Ma. The most easterly metasediment sample contains zircons with ages which cluster around 3007 ± 6 Ma which is within error of the age of a batholith dated in the adjacent Marmion Lake area of the Wabigoon Subprovince. This, along with the fact that most detrital zircons were only slightly abraded by sedimentary transport, suggests that the source of many zircons may have been proximal from the north. There is also a significant cluster of young zircon ages in the range 2698–2709 Ma. Surprisingly, only a few ages correspond to the time of most intense arc-type felsic volcanism (2710–2745 Ma) in the main greenstone belt of the western Wabigoon Subprovince. Sedimentary deposition occurred at a time of extensive deformation in the adjacent Wabigoon Subprovince. The pattern of detrital grain ages indicates that this involved uplift of a complex older sialic landmass. Petrographic evidence for the presence of substantial felsic volcanic detritus as well as the fact that many of the zircons only slightly pre-date deposition indicates that much of the sediment was probably derived from erosion of active felsic volcanic centers. These data combined with regional geochronological and field studies favour deposition in a basin formed by late orogenic collisional events.