The role of metamorphic fluids in the development of the Cornubian orefield: fluid inclusion evidence from south Cornwall

1990 ◽  
Vol 54 (375) ◽  
pp. 219-230 ◽  
Author(s):  
J. J. Wilkinson
Keyword(s):  
Vapour Phase ◽  
Ore Deposits ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Cation Composition ◽  
Molar Ratios ◽  
Low Salinity ◽  
Granite Emplacement ◽  
Granite Batholith ◽  
End Members

AbstractVeins developed during contact metamorphism associated with the emplacement of the Cornubian granite batholith contain both H2O-rich and CO2-rich fluid inclusions. Microthermometric data indicate that unmixing of a low-CO2, low-salinity fluid occurred at 400–200°C and 1000–500 bars to produce low-density CO2-rich vapour and saline aqueous fluids (8–42 wt. % NaCl equivalent). Decrepitation-linked ICP analyses show that the cation composition of the brines is dominated by Na, K and Ca, but that significant amounts of Li, Sr, Ba, Fe, Mn, Zn and B are also present. Bulk volatile analyses confirm the dominance of CO2 over N2 and CH4 in the vapour phase, with CO2/N2 molar ratios of 15.3–28.7 and CO2/CH4 molar ratios of 66.9–292. The relative abundance of nitrogen suggests an aureole-derived ‘organic’ component is present.The source of the fluids is ambiguous as they are intermediate in composition between ideal ‘magmatic’ and ‘metamorphic’ end-members. It is proposed that this is due to mixing of the two types of fluid in the contact aureole during granite intrusion. A model is suggested in which magmatic-metamorphic circulation occurred synchronously with granite emplacement and subsequently evolved to a meteoric-dominated system with the bulk of the ore deposits forming in response to the influx of meteoric fluids.

The greenschist facies of an Archean assemblage near Wawa, Ontario

1983 ◽  
Vol 20 (9) ◽  
pp. 1409-1420 ◽  
Author(s):  
Paul A. Studemeister
Keyword(s):  
Native Gold ◽  
Regional Metamorphism ◽  
Volcanic Rocks ◽  
White Mica ◽  
Greenschist Facies ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Metamorphic Fluid ◽  
Vein Quartz ◽  
Low Salinity

A stock of trondhjemite intrudes an Archean succession metamorphosed to the greenschist facies at Gutcher Lake, 30 km north of Wawa in Ontario. The stock is 4 km2 in plan and is partly enveloped by an aureole of epidote–amphibole hornfels up to 1 km wide. Within this aureole chlorite partly replaces biotite; chlorite, calcite, and quartz mantle hornblende and epidote; zoned amphibole has a rim of actinolite; and albite varies from 0 to 8% An. The stock has chlorite pseudomorphous after biotite, and feldspar is mottled by white mica and has a clear rim of albite. Fractures filled with quartz, calcite, ankerite, white mica, chlorite, pyrite, and native gold cross-cut the stock and its aureole. Wall rocks to these veins were modified by hydrothermal alteration with addition of Si, Fe, K, H2O + CO2, S, and Rb, leaching of Na, and a shift in Fe2+/Fet from ~0.66 to ~0.90. Primary inclusions in the vein quartz have a solution with a CO2 gas bubble that homogenizes into the liquid at around 300 °C.Initial contact metamorphism of volcanic rocks at T = 450–550 °C and P < 200 MPa (2 kbar) formed an aureole of epidote–hornblende hornfels near the stock. Subsequent regional metamorphism during the Archean at T = 325–450 °C and P = 200–300 MPa (2–3 kbar) retrograded the stock and its contact aureole to a lower greenschist assemblage. The retrogression involved hydration and CO2 fixation in hornfels and trondhjemite by a hot reducing fluid of low salinity. This metamorphic fluid precipitated native gold with quartz and pyrite along fractures in response to cooling and chemical reaction with wall rocks.

Shear zone development and structurally-controlled skarn ore mineralization in the Rosas district, SW Sardinia.

2021 ◽  
Author(s):  
Matteo Luca Deidda ◽  
Antonio Attardi ◽  
Fabrizio Cocco ◽  
Dario Fancello ◽  
Antonio Funedda ◽  
...  
Keyword(s):  
Shear Zone ◽  
Large Scale ◽  
Ore Deposits ◽  
Axial Plane ◽  
Contact Metamorphism ◽  
Upper Ordovician ◽  
Fold And Thrust Belt ◽  
Ore Mineralization ◽  
Map Scale ◽  
Stratigraphic Succession

&lt;p&gt;The Rosas Shear Zone (RSZ) is a 1 km thick brittle-ductile shear zone that outcrops in the Variscan fold and thrust belt foreland of SW Sardinia, where several important ore deposits were mined in the last century. The RSZ lies in the footwall and strikes parallel to the NE-dipping regional thrust that separates the Variscan foreland from the nappe zone. Two thrusts that developed along the limbs of two km-scale overturned antiforms, with NE-dipping axial plane, bound the RSZ. The folds show a SW-facing direction and a well-developed axial plane cleavage, and affect a lower Cambrian-upper Ordovician stratigraphic succession mainly made, from bottom to top, by a sequence about 200 m thick of dolostones and massive limestone followed by 50 m of marly limestones overlain by about 150 m of sandstones, pelites and siltstones, finally unconformable capped by conglomerates and siltstones, ranging in thickness from a few to 200 m. Differently, within the RSZ the bedding is completely transposed along the cleavage and its internal structure is characterized by anastomosing thrusts that affect the stratigraphic succession defining map-scale slices mainly consisting of dolostones and limestones embedded into the siliciclastic formations. It is noteworthy the occurrence of a NE-dipping, up to 100 m thick gabbro-dyke that postdates the deformation phases and that can be related to the exhumation of the chain during late Carboniferous-Permian times.&lt;/p&gt;&lt;p&gt;In the whole area, contact metamorphic and metasomatic processes selectively affected the Cambrian carbonate tectonic slices, originating several skarn-type orebodies. Mineralized rocks display the mineralogical assemblages and textures of Fe-Cu-Zn skarns, with relicts of anhydrous calcic phases related to the prograde metamorphic stage (garnet, clinopyroxene, wollastonite), frequently enclosed in a mass of hydrous silicates (actinolitic amphibole, epidote) and magnetite related to the retrograde metasomatic stage, in turn followed by chlorite, sulfides, quartz and calcite associated to the hydrothermal stage. Metasomatic reactions also involved mafic rocks, producing a mineral association marked by clinopyroxene, amphibole, epidote, prehnite and Ba-rich K-feldspar. Sulfide ores are made of prevailing sphalerite, chalcopyrite and galena, with abundant pyrite and pyrrhotite and minor tetrahedrite and Ag-sulfosalts. Garnets are andraditic/grossularitic, distinctly zoned and optically anisotropic. Field surveys pointed out the tight structural controls on skarn and ore formation. On a local scale, the gabbro emplacement along high- to low-angle NNW-SSE structures bordering the carbonate tectonic slices accentuate the effects of contact metamorphism, and metric to decametric mineralogical zonation (garnet&amp;#8594;pyroxene&amp;#8594;wollastonite) are recognized. On a larger scale, extensive hydrothermal fluid circulations involved the structures of the RSZ. Infilling of metasomatic fluids in carbonate tectonic slices is fault-controlled and aided by the increase in permeability due to the alteration of prograde silicates. The causative intrusion related to skarn ores belongs to the early Permian (289&amp;#177;1 Ma) ilmenite-series, ferroan granite suite which intrudes the RSZ about 3 km east from the studied area. The Fe-Cu-Zn skarn ores of Rosas are best interpreted as distal, structurally-controlled orebodies, connected to large-scale circulation of granite-related fluids in the km-sized plumbing system represented by the RSZ.&lt;/p&gt;

scholarly journals U-Pb monazite ages of the Kabanga mafic-ultramafic intrusions and contact aureoles, central Africa: Geochronological and tectonic implications

2019 ◽  
Vol 131 (11-12) ◽  
pp. 1857-1870 ◽  
Author(s):  
Jian-Wei Zi ◽  
Birger Rasmussen ◽  
Janet R. Muhling ◽  
Wolfgang D. Maier ◽  
Ian R. Fletcher
Keyword(s):  
Central Africa ◽  
Layered Intrusion ◽  
Thermal Anomaly ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
East African ◽  
Metamorphic History ◽  
History Of ◽  
Rodinia Assembly ◽  
Contact Aureoles

AbstractMafic-ultramafic rocks of the Kabanga-Musongati alignment in the East African nickel belt occur as Bushveld-type layered intrusions emplaced in metasedimentary sequences. The age of the mafic-ultramafic intrusions remains poorly constrained, though they are regarded to be part of ca. 1375 Ma bimodal magmatism dominated by voluminous S-type granites. In this study, we investigated igneous monazite and zircon from a differentiated layered intrusion and metamorphic monazite from the contact aureole. The monazite shows contrasting crystal morphology, chemical composition, and U-Pb ages. Monazite that formed by contact metamorphism in response to emplacement of mafic-ultramafic melts is characterized by extremely high Th and U and yielded a weighted mean 207Pb/206Pb age of 1402 ± 9 Ma, which is in agreement with dates from the igneous monazite and zircon. The ages indicate that the intrusion of ultramafic melts was substantially earlier (by ∼25 m.y., 95% confidence) than the prevailing S-type granites, calling for a reappraisal of the previously suggested model of coeval, bimodal magmatism. Monazite in the metapelitic rocks also records two younger growth events at ca. 1375 Ma and ca. 990 Ma, coeval with metamorphism during emplacement of S-type granites and tin-bearing granites, respectively. In conjunction with available geologic evidence, we propose that the Kabanga-Musongati mafic-ultramafic intrusions likely heralded a structurally controlled thermal anomaly related to Nuna breakup, which culminated during the ca. 1375 Ma Kibaran event, manifested as extensive intracrustal melting in the adjoining Karagwe-Ankole belt, producing voluminous S-type granites. The Grenvillian-aged (ca. 990 Ma) tin-bearing granite and related Sn mineralization appear to be the far-field record of tectonothermal events associated with collision along the Irumide belt during Rodinia assembly. Since monazite is a ubiquitous trace phase in pelitic sedimentary rocks, in contact aureoles of mafic-ultramafic intrusions, and in regional metamorphic belts, our study highlights the potential of using metamorphic monazite to determine ages of mafic-ultramafic intrusions, and to reconstruct postemplacement metamorphic history of the host terranes.

The western margin of the Coast Plutonic Complex on Hardwicke and West Thurlow Islands, British Columbia

1979 ◽  
Vol 16 (6) ◽  
pp. 1166-1175 ◽  
Author(s):  
Jo Anne Nelson
Keyword(s):  
Quartz Diorite ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Western Margin ◽  
Late Mesozoic ◽  
The North ◽  
Early Mesozoic ◽  
Plutonic Complex ◽  
Coast Plutonic Complex ◽  
Maximum Contact

The western margin of the Coast Plutonic Complex, one of the major tectonic boundaries of the Canadian Cordillera, has been variously interpreted as an intrusive contact, a shear zone, and a suture zone joining the Early Mesozoic Insular Belt to the North American continent. A representative section of this boundary, exposed on islands in Johnstone Strait, is an intrusive contact along which a quartz diorite with peripheral mafic phases truncates Early Mesozoic sediments and volcanics of the Insular Belt. Concordant hornblende–biotite pairs and two whole rock biotite isochrons date the intrusion as Late Jurassic (151 Ma). Prior to intrusion the stratified units underwent prehnite–pumpellyite facies metamorphism and west-northwest block faulting.The contact aureole of the quartz diorite and its associated mafic phases involves greenschist and hornblende–hornfels facies assemblages. Total pressure in the upper Karmutsen Formation during contact metamorphism was less than 2.5 × 105 kPa. The maximum contact temperature was between 670 and 700 °C. Forcible emplacement of the intrusion caused penetrative deformation of wall rocks in the inner aureole. The maximum contact temperatures indicate that the plutonic bodies were at near-liquidus temperatures when emplaced.The contact on Hardwicke and West Thurlow Islands appears representative of most of the tectonic boundary between the southern Coast Plutonic Complex and the Insular Belt. The western margin of the Coast Plutonic Complex is thus a Late Mesozoic magmatic front, the western limit of the intense magmatism that generated the Coast Plutonic Complex. The formation of Georgia Depression over the province boundary was a later event, coeval with major uplift of the Coast Plutonic Complex.

Chemical mineralogy of the aureole of the Nahant Gabbro at East Point, Nahant, Mass., USA

1979 ◽  
Vol 43 (326) ◽  
pp. 201-209 ◽  
Author(s):  
P. K. Verma
Keyword(s):  
Solid Solution ◽  
Experimental Work ◽  
Electron Microprobe ◽  
Lower Cambrian ◽  
Mineral Assemblage ◽  
Regional Metamorphism ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Low Grade ◽  
Solid Solution Series

SummaryThe Lower Cambrian Weymouth Formation at Nahant, Massachusetts, consisting of interbedded pelitic and calcareous rocks, was intruded by the Nahant Gabbro. The predominant metapelitic mineral assemblage of the contact aureole is quartz-muscovite-chlorite-magnetite-ilmenite. The calcareous hornfelses exhibit a varied mineral assemblage, and in some cases the variation can be spatially related to the intrusive. A number of cross-cutting thin mineral veins, many containing prehnite, are characteristically associated with these calcsilicate rocks.The minerals have been analysed by electron microprobe and this work indicates the presence of a possible solvus in the Fe3+-Al epidote solid solution series. At the physicochemical conditions of the formation of the Nahant hornfelses, the ferric mole fractions of coexisting epidotes are 0.49 and 0.98.Comparison with experimental work shows that the conditions of the contact metamorphism were T ≃ 500°C, Ptotal ≃ 2 kb, and XCO2 ≃ 0.2. However, the present assemblages are the result of a later low-grade regional metamorphism, the ultimate product of which was prehnite.

METAMORPHISM OF THE MEGUMA GROUP OF NOVA SCOTIA

1966 ◽  
Vol 3 (7) ◽  
pp. 959-974 ◽  
Author(s):  
F. C. Taylor ◽  
E. A. Schiller
Keyword(s):  
Nova Scotia ◽  
Regional Metamorphism ◽  
Greenschist Facies ◽  
Amphibolite Facies ◽  
Contact Metamorphism ◽  
Granitic Rocks ◽  
Structural Elements ◽  
Early Ordovician ◽  
Quartz Veins ◽  
Granite Emplacement

The Meguma group of lithic greywacke, feldspathic quartzite, slate siltstone, and argillite is Early Ordovician or older in age and has undergone both regional and contact metamorphism. Both types of metamorphism have resulted in recrystallization and locally in orientation of newly formed minerals. Metasomatism and retrogressive metamorphism are subordinate and only locally important. Regionally metamorphosed rocks are divided into greenschist and almandine–amphibolite facies, although some assemblages cannot be assigned with certainty. Locally, biotite and garnet isograds are mappable within the greenschist zone.Relationships between regional metamorphism and structural elements (folding) show that deformation preceded regional metamorphism. Intrusion of granitic rocks has produced a zone of contact metamorphism (hornblende–hornfels facies) that is superimposed upon regional greenschist facies rocks, which shows that granite emplacement occurred after the regional grade was reached. Gold–quartz veins are confined to areas lying in the greenschist zone of regional metamorphism, which suggests that the almandine–amphibolite zone is not favorable.

Metamorphosed Permian vertebrate fossils: geochemistry and mineralogy of “white” sharks

2020 ◽  
Author(s):  
Anthea Arns ◽  
Frank Tomaschek ◽  
Edith Alig ◽  
Katrin Weber ◽  
Hubert Vonhof ◽  
...  
Keyword(s):  
Inductively Coupled Plasma ◽  
Mineralogical Composition ◽  
Hydrothermal Activity ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Hydrothermal Conditions ◽  
Metamorphic Evolution ◽  
Shark Teeth ◽  
Icp Ms ◽  
Floodplain Sediments

&lt;p&gt;The intra-mountainous Saar-Nahe Basin (SNB), SW Germany, a strike-slip-fault basin formed during the Variscian orogeny, was filled by a large freshwater lake system during the Early Permian. The SNB experienced intense syn- and post-depositional magmatic activity, resulting in a complex volcano-sedimentary sequence of magmatic intrusions, lava flows and tephra deposits intercalating in continental red beds and limno-fluvial sediments. Fossils preserved in white color are found in Permo-Carboniferous fluvio-lacustrine siliciclastic floodplain sediments with thin intercalated limestone banks, of the Remigiusberg Formation in the SNB. The oldest amniote fossil of Germany and other partly articulated tetrapod remains were recovered from it at the Remigiusberg quarry near Kusel (e.g., Fr&amp;#246;bisch et al., 2011; Voigt et al. 2014). These terrestrial tetrapods were discovered together with aquatic vertebrate fossils in close proximity (&lt; 5 m; within the contact aureole) to an underlying decameter thick sill of kuselite, an auto-hydrothermally altered andesite. We aim to assess the thermal and chemical impact of post-depositional contact metamorphism and hydrothermal activity associated with this sill on the bioapatite of vertebrate skeletal remains by characterizing the elemental, isotopic and mineralogical composition of these fossils. White-colored, likely hydrothermally altered teeth of the freshwater shark Lebachacanthus were analyzed and compared to shark teeth of the same species retaining their original black color, from contemporaneous unmetamorphosed lacustrine black shale deposits in the SNB.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;In situ Electron Microprobe analysis and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) elemental profiles reveal distinct diagenetic histories for the black- and white-colored shark teeth. This is further supported by apatite &lt;span&gt;&amp;#7839;&lt;/span&gt;&lt;sup&gt;18&lt;/sup&gt;O and &lt;span&gt;&amp;#7839;&lt;/span&gt;&lt;sup&gt;13&lt;/sup&gt;C values, which indicate different secondary alteration by fluids for both facies. Raman spectroscopy and X-ray diffraction on bulk powder samples identify fluorapatite as the major mineral phase in all teeth. Apatite crystallinity of both dentin and enameloid is higher in white- than in black-colored fossils, consistent with crystallite growth due to thermal overprint of &gt; 500 &amp;#176;C. For both, white- and black-colored shark teeth, LA-ICP-MS U-Pb analyses yield inconclusive data and unexpectedly young ages inconsistent with known ages of deposition or metamorphism.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;We are currently analyzing the petrology of the kuselite to constrain the metamorphic evolution of sediments in the contact aureole by modelling. Additionally, heating experiments of modern bioapatite samples are performed to further constrain the alteration temperature. Altogether, these data will enhance our understanding of the particular thermometamorphic/hydrothermal conditions required to form white-colored, recrystallized vertebrate fossils in the context of the magmatic-metamorphic evolution of the SNB.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;FR&amp;#214;BISCH, J., SCHOCH, R.R., M&amp;#220;LLER, J., SCHINDLER, T. &amp; SCHWEISS, D.J. (2011): The oldest amniote from Germany: a sphenacodontid synapsid from the Saar-Nahe Basin. Acta Palaeontologica Polonica, 56: 113&amp;#8211;120.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;VOIGT, S., FISCHER, J., SCHINDLER, T., WUTTKE, M., SPINDLER, F. &amp; RINEHART, L. (2014): On a potential fossil hotspot for Pennsylvanian-Permian nonaquatic vertebrates in Europe. Freiberger Forschungshefte, C548: 39&amp;#8211;44.&lt;/p&gt;

Structural Evidence for a Palaeozoic Orogeny in North-west Malaya

1965 ◽  
Vol 102 (6) ◽  
pp. 501-520 ◽  
Author(s):  
B. N. Koopmans
Keyword(s):  
West Coast ◽  
Contact Metamorphism ◽  
Third Generation ◽  
The West ◽  
North West ◽  
Granite Intrusion ◽  
Crenulation Cleavage ◽  
Two Generations ◽  
Granite Batholith ◽  
Lower Palaeozoic

AbstractLower Palaeozoic rocks in the Langkawi Islands have been subject to three generations of deformation. In the slightly dynamo-meta-morph slates in the south-east two generations of cleavage folding have been developed with a N.N.W. trend. The first resulted in a recumbent folding with a subhorizontal slaty cleavage, whereas during the second generation the slaty cleavage was deformed in open folds and a crenulation cleavage developed in the steep limbs. The two deformations were probably not formed at exactly the same time, but seem to be related with each other. Boudinage of granite sills was contemporaneous with the development of the slaty cleavage.In the unmetamorphosed northern part of the area cross-folding is found. Flexure folds with a N.W. trend are superimposed on folds with a N.N.E. trend. In the Upper Palaeozoic sediments such deformations are missing. The Permian and Carboniferous strata show only a slight warping around the granite batholith which was intruded during Jurassic time (Pacific orogeny). A similar warping is found in the large low-angle overthrust, in which Silurian was thrust over Permo-Carboniferous. Consequently this thrust plane is dated as pre-granitic and post Permo-Carboniferous. Contact metamorphism by the granite intrusion has resulted in hornfelses in the Permo-Carboniferous sequence and in the development of oriented minerals, mainly clinozoisite, in the Ordovician-Silurian slates. Knick zones and intense jointing parallel with the mineral orientation (third generation structures) are also a result of the granite intrusion.The first two generations of folding, which are not present in the Upper Palaeozoic beds, are products of an erogenic phase, “the Langkawi folding phase,” during late Silurian-Devonian time. The lack of Devonian strata in Thailand and the incompleteness of the Devonian sequence in Malaya (recently a few outcrops of probably Middle-Upper Devonian strata are found along the west coast of Malaya) can be an indication that the Langkawi folding phase is not restricted to the Langkawi Islands.

scholarly journals Employing contact metamorphism to assess the conditions of pluton emplacement and timing of recrystallization in southwestern Kellys Mountain, Cape Breton Island, Nova Scotia

2017 ◽  
Vol 54 (11) ◽  
pp. 1165-1178 ◽  
Author(s):  
Nabil A. Shawwa ◽  
Robert P. Raeside ◽  
David W.A. McMullin ◽  
Christopher R.M. McFarlane
Keyword(s):  
Nova Scotia ◽  
Regional Metamorphism ◽  
Low Pressure ◽  
Contact Metamorphism ◽  
Contact Aureole ◽  
Low Grade ◽  
Cape Breton Island ◽  
Metasedimentary Rocks ◽  
Cape Breton ◽  
George River

At Kellys Mountain, Cape Breton Island, Nova Scotia, the late Neoproterozoic Glen Tosh formation (a low-grade metapsammite–metapelite unit of the George River Metamorphic Suite) has been intruded by diorite, granodiorite, and granite plutons, and the diorite hosts a narrow contact metamorphic aureole. New mapping and sampling in the contact aureole reveals that the metasedimentary rocks have reached amphibolite-facies metamorphism resulting in the development of neoformed biotite, muscovite, cordierite, ilmenite, garnet, andalusite, sillimanite, monazite, and spinel within the meta-pelite, a mineral assemblage also found in the Kellys Mountain Gneiss as a result of low-pressure regional metamorphism. Neoformed minerals and the disappearance of foliation defines a contact metamorphic aureole within 300 m of the pluton contacts. Petrographic and microprobe analyses of equilibrium assemblages in metapelitic units of the contact aureole yielded metamorphic pressures of 250 MPa, implying an intrusion depth of ∼9 km, with temperatures ranging from 365 to 590 °C. The presence of earlier-formed andalusite and garnet indicates the rocks may have initially undergone a low-pressure regional metamorphic event prior to contact metamorphism. Monazite in the contact aureole was dated using in-situ U–Pb methods and yielded an age of 480.9 ± 3.7 Ma, interpreted as the time of formation of the contact metamorphic aureole.

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