Complex ⁴⁰Ar/³⁹Ar age spectra from low metamorphic grade rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen

Low metamorphic grade rocks contain both detrital minerals and minerals newly grown or partly recrystallised during diagenesis and metamorphism. However, rocks such as these typically yield complex 40Ar/39Ar age spectra that can be difficult to interpret. In this study, we have analysed a suite of variably deformed rocks from a region of low metamorphic grade within the c. 514–490 Ma Delamerian Orogen, South Australia. The samples analysed range from siltstone and shale to phyllite and all contain either muscovite or phengite determined by hyperspectral mineralogical characterisation. Furnace step heating 40Ar/39Ar analysis produced complex apparent age spectra with multiple age components. Using the concept of asymptotes that define minimum and maximum ages for different components, we interpret the age spectra to preserve a range of detrital mineral ages, along with younger components related to either cooling or deformation- induced recrystallisation. Two samples contain Mesoproterozoic detrital age components, up to c. 1170 Ma, while the c. 515 Ma Heatherdale Shale which has both c. 566 Ma and c. 530 Ma detrital components. All samples contain younger lower (younger) asymptotes in the age spectra defined from multiple heating steps that range from c. 476 to c. 460 Ma. One interpretation of these younger ages is that they are caused by post-metamorphic cooling. However, the shape of the age spectra and the degree of deformation in the phyllites suggest the ages may record recrystallisation of detrital minerals and/or new mica growth during deformation. Potentially these c. 476 to c. 460 Ma ages suggest deformation in the upper portion of the orogen was facilitated by movement along regional faults and shear zones up to around 20 million years after the cessation of deformation in the high-metamorphic grade regions of the Delamerian Orogen.

Thermodynamic modelling of carbonate-silicate rocks from the Sakar unit, Sakar-Strandzha zone, SE Bulgaria

The P-T evolution of carbonate-bearing metasedimentary rocks from the Sakar unit (Sakar-Strandzha Zone, SE Bulgaria) has been obtained using Perple_X modelling and conventional geothermometry. The metamorphic conditions vary from greenschist facies (250–350 °C/2–4 kbar) in the Klokotnitsa village area to amphibolite facies (550–650 °C/4.5–6.5 kbar) in the Topolovgrad town area, confirming a general increase of the metamorphic grade at east-west direction.

Geochemistry evolution of Schists of northwest Obudu area southeastern Nigeria

Geochemistry of schists of Obudu area was carried out using ICP-MS and ICP-ES techniques in order to determine the geochemical evolution of the area. 40 samples were analyzed for their major, trace and REE composition. Field mapping revealed that gneisses, amphibolites and schists comprising migmatitic schists (MS), quartz-mica schists (QMS), garnet-mica schists (GMS), and hornblende biotite schists (HBS), intruded by granites, granodiorites, quartzofeldspathic rocks and dolerites occur in the area. Structural studies revealed that the schists trend approximately NE–SW (5 – 30o ) indicating the Pan-African event. Modal analysis revealed that the schists have average concentration of quartz (15vol.%), plagioclase (An45-19 vol.%), biotite (15vol.%), garnet (9.0vol.%) and muscovite (6vol.%), the remaining consists of accessory minerals. Geochemistry showed that all the schists have molecular Al2O3 > CaO+K2O+Na2O, indicating they are peraluminous metasedimentary pelites. Trace and REE element results show that all the analyzed schist samples are depleted in Hg, Ag, Be, Bi, and Sb below < 1.0ppm, but relatively enriched in Ba, Sr and Zr with average concentration of 996, 675.73, 243.13 ppm respective. The HREE are depleted with ΣHREE < 10.2, but the LREE are relatively enriched with ΣLREE > 289.54. The ΣLREE/ΣHREE ratio ranges from 9.17 to 33.4, with a large positive delta V at Eu. These findings indicate that the schists of Northwest Obudu area are highly fractionated and had attained at least the uppermost amphibolite metamorphic grade. The schists had contributed to the development of the Pan-African continent.

Contrasting mineralogy and strain partitioning across N-S oriented Sitampundi - Kanjamalai Shear in Sitampundi Anorthosite Layered Complex

Sitampundi Anorthosite Layered Complex (SALC) is a complexly folded and metamorphosed terrain that shows different metamorphic grade separated by a regional linear divide. In the north-eastern part of the complex, the anorthosites contain green-colored clinozoisites that are strikingly absent in the western part of the limb. Based on the presence of the clinozoisites, the entire SALC can be divided into two zones. The Sitampundi-Kanjamalai shear zone (SKSZ) separates mega crystals of clinozoite bearing anorthosites from clinozoisite free anorthosites. To add furthermore, strain analysis of different samples of anorthosite on either side of the zones was conducted by employing Flinn method. In general, anorthosites fall into the flattening field. The clinozoisite free anorthosites are more flattening and clinozoisite bearing anorthosites exhibit a slight difference in their strain ratio, ie., it is comparatively less flattening. Geochemistry of clinozoisites was studied using EPMA & XRD methods. The percentage of oxides obtained from EPMA coincides with that of epidote. But, XRD confirms the mineral to be clinozoisite indicating the transition phase of epidote to clinozoisite. Zoning has had occurred in clinozoisites with aluminium oxide rich core and FeO rich rim. This could be related to a retrogression corresponding to a shearing event.

Paleozoic evolution of crustal thickness and elevation in the northern Appalachian orogen, USA

The Acadian and Neoacadian orogenies are widely recognized, yet poorly understood, tectono-thermal events in the New England Appalachian Mountains (USA). We quantified two phases of Paleozoic crustal thickening using geochemical proxies. Acadian (425–400 Ma) crustal thickening to 40 km progressed from southeast to northwest. Neoacadian (400–380 Ma) crustal thickening was widely distributed and varied by 30 km (40–70 km) from north to south. Doubly thickened crust and paleoelevations of 5 km or more support the presence of an orogenic plateau at ca. 380–330 Ma in southern New England. Neoacadian crustal thicknesses show a strong correlation with metamorphic isograds, where higher metamorphic grade corresponds to greater paleo-crustal thickness. We suggest that the present metamorphic field gradient was exposed through erosion and orogenic collapse influenced by thermal, isostatic, and gravitational properties related to Neoacadian crustal thickness. Geobarometry in southern New England underestimates crustal thickness and exhumation, suggesting the crust was thinned by tectonic as well as erosional processes.

A metasedimentary source of gold in Archean orogenic gold deposits

The sources of metals enriched in Archean orogenic gold deposits have long been debated. Metasedimentary rocks, which are generally accepted as the main metal source in Phanerozoic deposits, are less abundant in Archean greenstone belts and commonly discounted as a viable metal source for Archean deposits. We report ultralow-detection-limit gold and trace-element concentrations from a suite of metamorphosed sedimentary rocks from the Abitibi belt and Pontiac subprovince, Superior Province, Canada. Systematic decreases in the Au content with increasing metamorphic grade indicate that Au was mobilized during prograde metamorphism. Mass balance calculations show that over 10 t of Au, 30,000 t of As, and 600 t of Sb were mobilized from 1 km3 of Pontiac subprovince sedimentary rock metamorphosed to the sillimanite metamorphic zone. The total gold resource in orogenic gold deposits in the southern Abitibi belt (7500 t Au) is only 3% of the Au mobilized from the estimated total volume of high-metamorphic-grade Pontiac sedimentary rock in the region (25,000 km3), indicating that sedimentary rocks are a major contributor of metals to the orogenic gold deposits in the southern Abitibi belt.

Protolith affiliation and tectonometamorphic evolution of the Gurla Mandhata core complex, NW Nepal Himalaya

Assigning correct protolith to high metamorphic-grade core zone rocks of large hot orogens is a particularly important challenge to overcome when attempting to constrain the early stages of orogenic evolution and paleogeography of lithotectonic units from these orogens. The Gurla Mandhata core complex in NW Nepal exposes the Himalayan metamorphic core (HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite, in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate that the HMC comprises Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS) rocks. Conventional interpretation of such provenance data would require the Main Central thrust (MCT) to be also outcropping within the core complex. However, new in situ U-Th/Pb monazite petrochronology coupled with petrographic, structural, and microstructural observations reveal that the core complex is composed solely of rocks in the hanging wall of the MCT. Rocks from the core complex record Eocene and late Oligocene to early Miocene monazite (re-)crystallization periods (monazite age peaks of 40 Ma, 25–19 Ma, and 19–16 Ma) overprinting pre- Himalayan Ordovician Bhimphedian metamorphism and magmatism (ca. 470 Ma). The combination of Sm-Nd isotopic analysis and U-Th/ Pb monazite petrochronology demonstrates that both GHS and LHS protolith rocks were captured in the hanging wall of the MCT and experienced Cenozoic Himalayan metamorphism during south-directed extrusion. Monazite ages do not record metamorphism coeval with late Miocene extensional core complex exhumation, suggesting that peak metamorphism and generation of anatectic melt in the core complex had ceased prior to the onset of orogen-parallel hinterland extension at ca. 15–13 Ma. The geometry of the Gurla Mandhata core complex requires significant hinterland crustal thickening prior to 16 Ma, which is attributed to ductile HMC thickening and footwall accretion of LHS protolith associated with a Main Himalayan thrust ramp below the core complex. We demonstrate that isotopic signatures such as Sm-Nd should be used to characterize rock units and structures across the Himalaya only in conjunction with supporting petrochronological and structural data.

Valentia Slate, Co. Kerry, Ireland: a Global Heritage Stone Resource proposal

&lt;div&gt;&lt;span&gt;Valentia Slate from the southwest of Ireland, is herein proposed as a Global Heritage Stone Resource. This Middle Devonian (Givetian) purple to pale green-coloured&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;fine-grained siltstone comprises the Valentia Slate Formation, part of the Old Red Sandstone which extensively crops out in southern Ireland.&amp;#160; The unit&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;which developed as an alluvial fan, has a thickness of over 3000m&amp;#160;&lt;/span&gt;&lt;span&gt;and shows a well developed cleavage and low metamorphic grade imposed during the Variscan which produced its slaty fabric. Although quarried from small surface openings from the late eighteenth century, the commercial value of certain horizons of the Valentia Slate Formation was first recognised by the local landowner The Knight of Kerry who commenced its extraction at Dohilla in 1816 for use as roofing slates.&amp;#160; The operation was expanded from the 1820s by the Hibernian Mining Company and later by the Valentia Slab Company and its successor&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;the Valentia Slate Company&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;which continued to quarry the stone until the late 1870s. Initally stone was extracted from surface workings but since 1840 it has been exclusively obtained from underground workings. From the 1880s the quarry went into decline due to competition from Wales and extraction ceased altogether in 1911 following a large rockfall at the opening to the quarry.&amp;#160; It was revived in the 1980s and recent investment has resulted in&amp;#160;&lt;/span&gt;&lt;span&gt;being able to provide &lt;/span&gt;&lt;span&gt;this quality stone to widespread markets. Although not easily split into thin slates Valentia Slate was first used locally for roofing and general building. However&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;as it could be cut into slabs of a variety of thicknesses and lenghts of up to 3m it was more readily adopted&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;both nationally and internationally&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;for use in buildings for window cills, steps, domestic fittings in bathrooms and kitchens, and paving both externally and internally as in the Houses of Parliament in London, the Paris Opera House, and for flagging in&amp;#160;&lt;/span&gt;&lt;span&gt;a &lt;/span&gt;&lt;span&gt;number of British railway termini.&amp;#160; The stone &lt;/span&gt;&lt;span&gt;was susceptible to&amp;#160;&lt;/span&gt;&lt;span&gt;and held sharp carving, and it it was also fabricated into headstones, memorials, garden furniture, and shelving. Stone was even exported in the 1870s to Brazil for use as railway sleepers. Craftsmen also&amp;#160;&lt;/span&gt;&lt;span&gt;fabricated&amp;#160;&lt;/span&gt;&lt;span&gt;lamps&amp;#160;&lt;/span&gt;&lt;span&gt;and&amp;#160;&lt;/span&gt;&lt;span&gt;birdhouses from the material and its most celebrated use was for billiard and snooker tables, a number of&amp;#160;&lt;/span&gt;&lt;span&gt;which&amp;#160;&lt;/span&gt;&lt;span&gt;were highly decorative having been enamelled.&amp;#160; During the height of production over 500 men were employed quarrying and working Valentia Slate. The first tramway in an Irish quarry was installed in about 1816 and was used to transport stone and sawn slabs from the quarry to Knightstown&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;some 4km away&lt;/span&gt;&lt;span&gt;,&amp;#160;&lt;/span&gt;&lt;span&gt;where it was further fabricated if required in a dedicated stoneyard prior to exportation from the adjacent slate quay.&amp;#160; Today extraction continues and the stone is used for a variety of restoration, decorative and construction purposes. The longevity of its extraction, its versatility of use, and the extent of the exportation of the Valentia Slate makes it worthy to be proposed as a Global Heritage Stone Resource.&lt;/span&gt;&lt;/div&gt;

Reconstructing the Ediacaran to Ordovician history of the Baltoscandian Margin of continent Baltica

&lt;p&gt;In the Scandes, the lower thrust sheets of the Caledonian allochthons provide unambiguous stratigraphic evidence of correlation with the successions of the Baltoscandian platform. Cambrian successions, including the Alum Shale Formation, providing the footwall for the main Caledonian decollement in Scandinavia, can be followed at least 200 km westwards from the thrust front into the hinterland of the orogen. The overlying early Palaeozoic strata provide evidence of facies changes into foreland basin deposits in the mid Ordovician and early Silurian; also of Ediacaran and Cryogenian successions, including Marinoan tillites. The amount of internal shortening in the Lower Allochthon is not uncontroversial, but certainly amounts to more than 100 km, implying that all the overlying alllochthons in the Scandes were derived from west of the Norwegian coast.&lt;/p&gt;&lt;p&gt;The metamorphic grade of the units in the Lower Allochthon increases from low to high greenschist facies, from the thrust front westwards into the deep hinterland. Overlying thrust sheets of the Middle Allochthon are of higher metamorphic grade and more ductilely deformed. The basal parts are usually dominated by basement-derived units and Neoproterozoic sedimentary rocks. They are overthrust by dolerite dyke-intruded thrust sheets, the S&amp;#228;rv Nappes, with host-rocks dominated by Cryogenian and Ediacaran sandstones, the former including subordinate limestones and Marinoan tillites. The Baltoscandian margin dolerite dyke swarms amount to up to c. 35% of these thrust sheets.&lt;/p&gt;&lt;p&gt;The overlying, highest tectonic units in the Middle Allochthon (the Seve Nappe Complex, SNC) are of amphibolite and higher metamorphic grade. They include a greater variety of lithologies, including some that are very similar to those in the underlying S&amp;#228;rv Nappes (e,g. quartzites and eclogitized dolerites). The metasedimentay host rocks include a wide range of paragneisses and marbles. Abundant mafic rocks include metamorphosed gabbros, basalts and peridotites and, together with the dyke swarms, can totally dominate the composition of some thrust sheets. The similar geochemistry and early Ediacaran age (c. 600 Ma) of the mafic rocks in the S&amp;#228;rv and Seve nappes define the Baltoscandian outermost margin and continent-ocean transition zone (COT). Iapetus Ocean terranes comprise the overlying thrust sheets of the Upper Allochthon (e.g. the K&amp;#246;li Nappe Complex).&lt;/p&gt;&lt;p&gt;The metamorphism of the different thrust sheets in the SNC provide clear evidence that some parts were subducted; others not. A wide range of isotope age data constrain the timing of subduction, with the earliest ages in the mid Cambrian (c. 505 Ma) to early Ordovician (c. 483 Ma). It has been suggested that the deposition of the Alum Shale Formation on the Baltscandian platform, was related to this early Caledonian subduction. A more probable interpretation is that subduction along the outermost edge of this highly extended COT did not influence the edge of the platform till the early Tremadoc.&lt;/p&gt;&lt;p&gt;Some authors have introduced cryptic sutures into the Baltoscandian outer margin, described above. They should reassess their data and better define the evidence for their conviction.&lt;/p&gt;