Syntectonic Leucogranites Research Articles

Timing and duration of meteoric water infiltration in the Quiberon detachment zone (Armorican Massif, Variscan belt, France)

Assessing the geochemical signature and the role of fluids in a key Variscan detachment zone demonstrates the link between crustal deformation, thermo-mechanical events and Variscan mineralization. We document meteoric fluid infiltration into the ductile segment of the Late-Carboniferous Quiberon detachment zone (QDZ), when synkinematic muscovite and tourmaline crystallized and equilibrated with deuterium-depleted surface-derived fluids during high-temperature deformation. Titanium-in-muscovite thermometry supported by microstructures indicate that syntectonic isotope exchange between fluids and hydrous minerals occurred above 500 °C. 40Ar/39Ar muscovite data (∼319–∼303 Ma) and U(–Th)/Pb geochronology on zircon, monazite and apatite (∼318–∼305 Ma) from syntectonic leucogranites together with microstructural and geochemical (U and REE contents) data suggest that meteoric fluid-rock-deformation interaction started at ∼320 Ma and played a major role in leaching uranium at ∼305 Ma. U–Th/Pb data (∼330–∼290 Ma) from migmatites located below the QDZ strengthen the idea that meteoric fluid infiltration, detachment activity, syntectonic leucogranite emplacement and migmatization were coeval and allowed the development of a sustained hydrothermal system.

Fluid inclusion 40Ar/39Ar geochronology of andalusite from syn-tectonic quartz veins: New perspectives on dating deformation and metamorphism in low-pressure metamorphic belts

Syn-tectonic quartz veins are widely present in low- to medium-grade metapelitic belts, however, dating these veins is challenging due to the lack of suitable target minerals for most geochronological methods. Andalusite is a common K-poor aluminosilicate mineral in these quartz veins. In this study, we demonstrate the feasibility of dating fluid inclusions in andalusite using the 40Ar/39Ar gentle stepwise crushing technique. Three andalusite samples in syn-tectonic quartz veins from the Chinese Altai, Central Asia yielded similar gas release patterns. The first crushing steps generated significant excess 40Ar which likely originated from secondary gas inclusions. These steps gave anomalously old and quickly declining apparent ages. The following crushing steps generated less significant excess 40Ar and yielded concordant and well-defined Early Permian ages of 282–277 Ma. These steps were likely dominated by the degassing of intra-crystal liquid-rich fluid inclusion planes. Two of the crushed powder samples from the stepwise crushing experiments were selected for additional 40Ar/39Ar stepwise heating using a furnace and yielded a much younger age of 216 Ma. Laser stepwise heating of muscovite flakes in two of the investigated andalusite samples and one chloridized biotite sample from a different andalusite-bearing quartz vein yielded 40Ar/39Ar ages of 243–241 Ma and 214 Ma, respectively. Compared with gases released from the crushing experiments, gases extracted from the heating experiments showed distinct Cl–K–Ca compositional characteristics and younger ages with near atmospheric initial 40Ar/36Ar value. These observations suggest that 1) contrary to common wisdom, excess 40Ar in K-poor nesosilicates are preferably preserved in fluid inclusions rather than mineral lattices, and 2) K (Ar) contamination from K-bearing solid phases is negligible during gentle stepwise crushing. As such, the 282–277 Ma fluid inclusion 40Ar/39Ar ages could be considered as the timing of andalusite growth in the quartz veins. The younger muscovite 40Ar/39Ar ages likely record more recent cooling events. The youngest 40Ar/39Ar ages of the crushed andalusite powder and the biotite sample could reflect the timing of sericitization and chloritization of the samples, which is evidenced by petrologic observations and x-ray K mapping results. Furthermore, andalusite 40Ar/39Ar ages agree with the monazite/zircon U–Pb ages (280–273 Ma) of syn-tectonic leucogranite and pegmatite dykes and metamorphic ages (299–262 Ma) for the low-pressure metamorphic rocks of the region. These results suggest that fluid inclusion 40Ar/39Ar geochronology of andalusite can be used to constrain the timing of deformation and associated metamorphism in low-pressure metapelitic terranes, which can also provide more complete pictures of the tectono-thermal evolution in combination with traditional 40Ar/39Ar stepwise heating geochronology.

Geochronological constraints on the geological history and gold mineralization in the Tick Hill region, Mt Isa Inlier

The Tick Hill Gold deposit in the southern Mary Kathleen Domain of the Mount Isa Inlier is hosted in a strongly deformed, Paleoproterozoic volcano-sedimentary sequence intruded by pre- and syn-tectonic granites. Igneous rocks and quartzite from the Tick Hill region were dated to constrain the age of the lithologies, deformation events, and gold mineralization. LA-ICP-MS, U-Pb zircon ages for these rocks, together with field relationships, confirm the presence of: (1) 1855–1850 granite belonging to the Kalkadoon Supersuite at ∼10 km and ∼4 km west of Tick Hill, respectively; (2) 1790–1770 Ma early syn-tectonic granite along the contact zone between mapped Argylla Formation and the Kalkadoon Supersuite west of Tick Hill, and later syn-tectonic leucogranite within the immediate vicinity of Tick Hill, including the host rocks to gold mineralization; and (3) late-tectonic, 1525–1520 Ma pegmatite and associated hydrothermal activity in the Tick Hill area that resulted in the mobilization of gold. Quartzite ridges in the hanging wall and footwall of the orebody provided contrasting results, with the youngest zircon population groupings at ca. 1781 Ma and 1841 Ma, respectively. Textural evidence suggests that much of the hanging wall quartzite is probably metasomatic in origin, and the ca. 1781 Ma age group, which was derived from mostly prismatic, euhedral zircon, reflects the age of a heavily silicified quartzofeldspathic gneiss. Thus, the age of the youngest detrital zircon group, ca. 1841 Ma, constrains the maximum age of the sequence at Tick Hill. Field evidence suggests that the 1790–1770 Ma granites intruded into the sedimentary sequence that hosts gold mineralization, indicating that the supracrustal rocks are at least ca. 1790 Ma in age and should not be grouped as Corella Formation. These sediments were affected by intense shearing and upright folding between 1790 Ma and 1770 Ma and younger normal faulting and metasomatism around 1525–1520 Ma. Early gold was introduced during D1 peak metamorphism at 1790–1770 Ma while the later mineralizing events involved the new introduction of gold or the remobilization of pre-existing older gold around 1525–1520 Ma. One of the major outcomes of this study is that the old schist zones in the south Mary Kathleen Domain are prospective for gold mineralization.

Late Carboniferous paleoelevation of the Variscan Belt of Western Europe

We present the first stable isotope paleoaltimetry estimates for the hinterland of the eroded Variscan Belt of Western Europe based on the hydrogen isotope ratios of muscovite from syntectonic leucogranites that have been emplaced at ∼315 Ma. We focus on the Limousin region (Western Massif Central, France) where peraluminous granites are spatially associated with strike-slip and detachment shear zones that developed as a consequence of Late Carboniferous syn- to post-orogenic extension. In this region, we show that the north-east corner of the Millevaches massif (located at the junction between brittle and ductile fault systems) represented a pathway for Earth surface-derived fluids that penetrated the crust and reached the ductile segment of the low-angle Felletin detachment zone. Using microstructural, thermometry, hydrogen isotope and 40Ar/39Ar geochronological data, we show that these Variscan meteoric fluids interacted with hydrous silicates during high temperature deformation between ∼318 and 310 Ma. For paleoaltimetry purposes, we reference our hydrogen isotope record (δD) of ancient meteoric fluids from mylonitic rocks to ∼295 Myr-old records retrieved from freshwater shark remains preserved in the Bourbon l'Archambault basin that developed in the external zones of the orogen. A ∼76‰ difference in δDmeteoric water values between the Millevaches massif (δDmeteoric water value = −96 ± 8‰) and the Bourbon l'Archambault foreland basin (δDwater value = −20 ± 6‰) is consistent with paleoaltimetry estimates of 3.4 ± 0.7 km based on a modern lapse rate of ∼-22‰/km for δDwater values. The rather large difference in δD values between the foreland basin and the continental interior suggests that the hinterland of the Variscan belt of western Europe was high enough to act as a barrier to moisture transport from the south-south-east and induce an orographic rain shadow to the north.

Oligocene initiation of the South Tibetan Detachment System: Constraints from syn-tectonic leucogranites in the Kampa Dome, Northern Himalaya

Himalayan leucogranites provide a robust record of the thermal and tectonic processes that occurred during convergence of the Eurasian and Indian tectonic plates. We present structural, petrological, mineralogical, whole-rock geochemical, SrNd isotope, and geochronological results for Cenozoic leucogranites from the Kampa Dome, Northern Himalaya. The Kampa leucogranites are typical syn-tectonic granites and occur as a series of sills, dikes, and puddings in the Kampa Shear Zone, which is considered to be the exhumed portion of the South Tibetan Detachment System (STDS) in the Northern Himalaya. The Kampa syn-tectonic leucogranites provide information about the timing and nature of regional kinematics, and record thermal conditions and the initiation of the STDS. Whole-rock geochemistry and SrNd isotope analyses show that the Kampa leucogranites are characterized by the following: (1) they are peraluminous and have high SiO2, Al2O3, and A/CNK, low TiO2, MgO, P2O5, and TFe2O3, and variable CaO, Na2O, K2O, and total alkalis; (2) they are enriched in large-ion lithophile elements (e.g., K and Rb) and depleted in high-field-strength elements (e.g., Zr and Ti), with negative Eu anomalies; and (3) they display variable initial 87Sr/86Sr compositions (0.750758-0.826065), and their εNd(i) values range between −7.6 and −12.2. Anatexis of graywackes and metapelites in the Great Himalayan Crystalline Series played an important role in the formation of the Kampa leucogranites. Anatexis was controlled by fluid-absent partial melting of muscovite. Mineralogical and petrological analyses show that zircons belong to an early crystallization phase, whereas monazites are remnants of the final crystallization phase of the granitic melt. Zircon UPb dating reveals that anatexis in the deep Himalayan crust occurred at ca. 32–23 Ma, during the Oligocene. Monazites yield homogeneous 208Pb/232Th ages of ca. 23.5 Ma and record rapid emplacement in the Kampa Shear Zone. These results indicate that movement on the STDS was initiated during the early Oligocene and became well-developed during the late Oligocene in the Kampa region. The Oligocene was a unique period of counterbalance between continued plate convergence and intense extension in the Himalayan Orogen.

Meteoric fluid‐rock interaction in Variscan shear zones

AbstractVariscan shear zones in the Armorican Massif represent sites of strong fluid‐rock interaction. The hydrogen isotope composition of muscovite (δDMs) from syntectonic leucogranite allows to determine the source of fluids that infiltrated the footwall of three detachment zones and the South Armorican Shear Zone. Using temperatures of hydrogen isotope exchange estimated from microstructural data, we calculate the hydrogen isotope ratios of water (δDwater) present within the shear zones during high‐temperature deformation. A ~40‰ difference in δDwater values from deep to shallow crustal level reveals a mixing relationship between deep crustal fluids with higher δD values that range from −34 to −33‰, and meteoric fluids with δD values as low as −74‰ in the upper part of detachment footwalls.

The Geological Evolution of the Variscan Jebilet Massif, Morocco, Inferred From New Structural and Geochronological Analyses

AbstractThe present work aims at understanding the tectonic evolution of the Jebilet massif, Morocco, during the Late Paleozoic as constrained by structural, metamorphic, and geochronological studies. From Late Devonian to Early Carboniferous, bordering faults controlled the opening of the Jebilet intracontinental basin (D0 stage) as shown by sedimentary infill. This episode was accompanied by a magmatic activity, newly dated between 358 ± 7 Ma and 336 ± 4 Ma. The first record of the Variscan event affected the Jebilet by the Late Visean‐Namurian and is represented by allochthonous superficial nappes emplaced at shallow depth in a moderately lithified sedimentary succession. D1 also developed regional‐scale recumbent folds trending E‐W that may suggest N‐S crustal shortening not generating crustal thickening nor contributing to metamorphism. The main Variscan D2 episode consists of a progressive evolution from bulk coaxial deformation to noncoaxial dextral transpression consistent with NW‐SE horizontal shortening, resulting in a moderate thickening. This episode was accompanied by HT‐LP metamorphism and syntectonic intrusions controlled by an inherited thermal anomaly in relation with the intracontinental rifting stage (D0). Based on previous age determinations from syntectonic leucogranite and metamorphic rocks, D2 is dated between 310 and 280 Ma. The tectono‐metamorphic evolution of the Jebilet massif can be correlated with a plate‐tectonic scenario evolving from, first a Late Devonian‐Early Carboniferous basin formation during stretching of the north‐Gondwana margin and initiation of the Paleotethys Ocean, and, second, to a Late Carboniferous‐Early Permian ocean closure (Rheic or Paleotethys Oceans depending of scenarios) that resulted in the final Variscan‐Alleghanian tectonics.

Tectonic activity along the inner margin of the South Tibetan detachment constrained by syntectonic leucogranite emplacement in Western Bhutan

In Western Bhutan Himalayas leucogranite dykes emplaced in sub-vertical hybrid fractures that cut across the high-grade rocks of the upper Greater Himalayan Sequence just below to the South Tibetan Detachment. The granitic dykes dip to the north often showing a mylonitic deformation with a top-down to-the-N sense of shear. The high-angle fractures are interpreted to be related to the evolution of the South Tibetan Detachment toward a brittle regime of deformation. U-Pb monazite ages constrain the leucogranite emplacement at 13.9±06 Ma implying that brittle-ductile deformation of the South Tibetan Detachment was active at that time. NNE-SSW to nearly E-W trending large scale antiforms and synforms mapped in NW Bhutan affected the Greater Himalayan Sequence and South Tibetan Detachment only after 14 Ma.

Magmatic and hydrothermal behavior of uranium in syntectonic leucogranites: The uranium mineralization associated with the Hercynian Guérande granite (Armorican Massif, France)

Most of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous leucogranite was emplaced in an extensional deformation zone at ca. 310Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave.Airborne radiometric data and published trace element analyses on the Guérande leucogranite suggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment in the apical zone of the granite likely occurred during both fractional crystallization and the interaction with magmatic fluids. The low Th/U values (<2) measured on the Guérande leucogranite likely favored the crystallization of magmatic uranium oxides. The oxygen isotope compositions of the Guérande leucogranite (δ18Owhole rock=9.7–11.6‰ for deformed samples and δ18Owhole rock=12.2–13.6‰ for other samples) indicate that the deformed facies of the apical zone underwent sub-solidus alteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz comb from a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1–6wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trapping temperatures in the range of 250–350°C suggest an elevated geothermal gradient, probably related to regional extension and the occurrence of magmatic activity in the environment close to the deposit at the time of its formation. U-Pb dating on uranium oxides from the Pen Ar Ran and Métairie-Neuve deposits reveals three different mineralizing events. The first event at 296.6±2.6Ma (Pen Ar Ran) is sub-synchronous with hydrothermal circulations and the emplacement of late leucogranitic dykes in the Guérande leucogranite. The two last mineralizing events occur at 286.6±1.0Ma (Métairie-Neuve) and 274.6±0.9Ma (Pen Ar Ran), respectively. Backscattered uranium oxide imaging combined with major elements and REE geochemistry suggest similar conditions of mineralization during the two Pen Ar Ran mineralizing events at ca. 300Ma and ca. 275Ma, arguing for different hydrothermal circulation phases in the granite and deposits. Apatite fission track dating reveals that the Guérande granite was still at depth and above 120°C when these mineralizing events occurred, in agreement with the results obtained on fluid inclusions at Pen Ar Ran.Based on this comprehensive data set, we propose that the Guérande leucogranite is the main source for uranium in the Pen Ar Ran and Métairie-Neuve deposits. Sub-solidus alteration via surface-derived low-salinity oxidizing fluids likely promoted uranium leaching from magmatic uranium oxides within the leucogranite. The leached out uranium may then have been precipitated in the reducing environment represented by the surrounding black shales or graphitic quartzites. As similar mineralizing events occurred subsequently until ca. 275Ma, meteoric oxidizing fluids likely percolated during the time when the Guérande leucogranite was still at depth. The age of the U mineralizing events in the Guérande region (300–275Ma) is consistent with that obtained on other U deposits in the EHB and could suggest a similar mineralization condition, with long-term upper to middle crustal infiltration of meteoric fluids likely to have mobilized U from fertile peraluminous leucogranites during the Late Carboniferous to Permian crustal extension events.

Strain distribution within a km-scale, mid-crustal shear zone: The Kuckaus Mylonite Zone, Namibia

The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic (and minor mafic) gneisses which had previously experienced granulite facies metamorphism during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also deformed in the shear zone. Pre-KMZ deformation (D1) is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the edge of leucogranites and between gneissic lozenges preserving D1 migmatitic foliations. Strain localization appears controlled by pre-existing foliations, grain size, and compositional anisotropy between leucogranite and granulite. The local presence of retrograde minerals indicate that fluid infiltration occurred in places, but most ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition and D1 fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space, with deformation localization along planes of rheological contrast and along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale ductile shear zones may be highly dependent on lithology and the orientation of pre-existing fabric elements. In addition, foliation development and grain size reduction in high strain zones further localizes strain during progressive shear, maintaining the anastomosing shear zone network established by the pre-existing heterogeneity.

Differentiation of peraluminous leucogranites “en route” to the surface

Leucogranites display petrological and geochemical heterogeneities that can be related to either primary processes in the source of melts or to secondary processes such as fractional crystallization. In the Armorican Massif, a Hercynian domain in western France, syntectonic leucogranites were emplaced along major shear zones, the so-called South Armorican Shear Zone. The Lizio and Questembert massifs are derived from a similar metasedimentary source ( ε Nd(T) between − 4 and − 6) and have similar magmatic volumes. They display, however, distinct depths of emplacement, with Questembert being emplaced ca. 5–10 km shallower than Lizio. The shallower emplacement of Questembert is recorded by contact metamorphic assemblages and the levels of intrusion in correlated sedimentary sequences. The Questembert magmas are more differentiated than the Lizio ones, as demonstrated by a lower amount of modal biotite, a higher SiO 2 content (72.8–74.2 wt.% vs. 71.7–73.3 wt.%) and a more pronounced peraluminous character. The high-SiO 2 Questembert magmas can be derived from the low-SiO 2 Lizio ones by 15–20 wt.% fractionation of the assemblage alkali feldspar + plagioclase + biotite. Fractionation of zircon and monazite, hosted in biotite, is indicated by a systematic decrease in Zr and Th + LREE contents. The magmatic evolution is well recorded by the oxygen isotope evolution of whole rocks and mineral separates. The Questembert massif records evidence of a two stage fluid–rock interaction. A high-T stage corresponds to the magmatic fluid exsolution, and is recorded in the most evolved samples: appearance of tourmaline in the matrix, changes in the composition of muscovite, increase in whole rock Be, Sn, Rb and W contents, fractionation of K/Rb, Zr/Hf and Nb/Ta ratios, and fractionation of REE patterns. A second post-solidus alteration is indicated by oxygen isotope disequilibrium between quartz and feldspar in the Questembert massif and requires the circulation of low- δ 18O fluids, likely derived from the surface. We propose that the distinct petrological, mineralogical and geochemical evolutions of the Questembert and Lizio magmas are related to processes acting during the ascent of magmas through the continental crust. Questembert is more differentiated than Lizio because it covered a greater distance during its vertical migration, enhancing crystal fractionation from melts.

40Ar/ 39Ar thermochronology of the Kampa Dome, southern Tibet: Implications for tectonic evolution of the North Himalayan gneiss domes

Structural and thermochronological studies of the Kampa Dome provide constraints on timing and mechanisms of gneiss dome formation in southern Tibet. The core of Kampa Dome contains the Kampa Granite, a Cambrian orthogneiss that was deformed under high temperature (sub-solidus) conditions during Himalayan orogenesis. The Kampa Granite is intruded by syn-tectonic leucogranite dikes and sills of probable Oligocene to Miocene age. Overlying Paleozoic to Mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite + staurolite grade at the base of the sequence to unmetamorphosed at the top. The Kampa Shear Zone traverses the Kampa Granite — metasediment contact and contains evidence for high-temperature to low-temperature ductile deformation and brittle faulting. The shear zone is interpreted to represent an exhumed portion of the South Tibetan Detachment System. Biotite and muscovite 40Ar/ 39Ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14.22 ± 0.18 to 15.54 ± 0.39 Ma cooling ages and concordant spectra with 14.64 ± 0.15 to 14.68 ± 0.07 Ma cooling ages. Petrographic investigations suggest disturbed samples are associated with excess argon, intracrystalline deformation, mineral and fluid inclusions and/or chloritization that led to variations in argon systematics. We conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at ∼ 14.6 Ma. The Kampa Granite yields the youngest biotite 40Ar/ 39Ar ages of ∼ 13.7 Ma immediately below the granite–metasediment contact. We suggest that this age variation reflects either varying mica closure temperatures, re-heating of the Kampa Granite biotites above closure temperatures between 14.6 Ma and 13.7 Ma, or juxtaposition of rocks with different thermal histories. Our data do not corroborate the “inverse” mica cooling gradient observed in adjacent North Himalayan gneiss domes. Instead, we infer that mica cooling occurred in response to exhumation and conduction related to top-to-north normal faulting in the overlying sequence, top-to-south thrusting at depth, and coeval surface denudation.

A newly discovered orogenic event in Morocco: Neoproterozic ages for supposed Eburnean basement of the Bou Azzer inlier, Anti-Atlas Mountains

The Bou Azzer inlier within the Anti-Atlas orogenic belt of Morocco exposes a dismembered ophiolite, long considered to mark a Late Neoproterozoic suture between the West African Craton (WAC) in the south, and Neoproterozoic arcs to the north. Southern parts of the Bou Azzer inlier include meta-igneous units, paragneiss, schist and muscovite-bearing granite that have until now been regarded as the c. 2 Ga Eburnean WAC basement onto which the ophiolite was obducted. An example of supposed Paleoproterozoic basement occurs at Tazigzaout. The main units within this complex are metagabbro, augen granite gneiss, finely banded granite gneiss and variably sheared syn-tectonic leucogranite, all of which are intercalated within a complex sub-vertical, upper greenschist facies, ductile dextral shear zone. The meta-igneous rocks are crosscut by a sheeted muscovite-biotite granite pluton. Three concordant U–Pb analyses of zircon from an augen granite gneiss provide a date of 753 +1/−2 Ma. Zircon from a nearby metagabbro provide an indistinguishable date of 752.2 ± 2.4 Ma. Both dates are considered best estimates of the crystallization ages of their igneous protoliths. Analyses of zircon from two crosscutting leucogranite bodies provide younger dates of 705 +2/−3 Ma and 701 +2/−1 Ma. The initial ɛ Nd values of all lithological units in the complex range from +4.9 to +6.0, with depleted mantle model ( T DM) ages of 890–790 Ma. The high, positive ɛ Nd values for all of these igneous units is indicative of an isotopically juvenile source, such as depleted-mantle derived material, and precludes substantial interaction with old, evolved crustal sources. These data demonstrate that the “basement” rocks of the Tazigzaout complex are Neoproterozoic intrusions, not c. 2 Ga Eburnean units, as found in more southerly Anti-Atlas inliers. The structural relationships and U–Pb ages are interpreted as recording dextrally transpressive deformation of c.755 Ma juvenile crust after c. 750 and before 700 Ma. This Neoproterozoic deformation occurred as much as 100 Ma prior to the top-to-the-north thrusting of the meta-igneous units over units of the ophiolite and arc, and provides strong evidence for two discrete ‘Pan African’ events in Morocco.

Late Neoproterozoic/Early Palaeozoic events in central Dronning Maud Land and significance for the southern extension of the East African Orogen into East Antarctica

Abstract New SHRIMP zircon data from Gjelsvikfjella and Muhlig-Hofmann-Gebirge indicate that the metamorphic basement is composed of Grenville-age rocks that are most likely part of the northeastern continuation of the Namaqua-Natal-Maud Belt. High-grade overprinting occurred between c. 560 and 490 Ma. This period is considered to bracket the collision of E- and W-Gondwana along the East African/Antarctic Orogen (EAAO) and also the subsequent collapse of the orogen. The onset of collision probably predates our oldest dates. We can clearly differentiate between an early compressional stage (Pan-African I) and a later dilational episode (Pan-African II). The Pan-African I event is characterised by the intrusion of leucogranites, and the formation of medium-U metamorphic zircon overgrowth. A sample of a syn-tectonic leucogranite gave a well-constrained crystallisation age of 558.4±5.6 Ma. These rocks are intensely deformed and have formed a second gt-bearing leucosome after their emplacement. Identical ages come from low-U metamorphic zircons and low-U rims from another leucosome at Festninga (557±13 Ma). The Pan-African II event (c. 530 and 490 Ma) is characterised by widespread melting and only little deformation. It is associated with charnockite magmatism, charnockitisation and high-temperature metamorphism. Wide, very high-U zircon rims form during this period. The early Pan-African II overprint between c. 530 and 520 Ma is recorded in almost all samples. An undeformed mafic dyke with a zircon crystallisation age of 523.2±4.8 indicates that there is no or minor deformation after its intrusion. This strongly metasomatised dyke, however, also indicates that although deformation ceased, intense fluid circulation continued. The youngest crystallisation ages come from a post-tectonic granite sheet which yields a crystallisation age of 486.9±3.8 Ma. Final cooling to temperatures below c. 600 °C is recorded by a titanite age of c. 483±11 Ma from the Stabben gabbro. Late to post-tectonic granitoids are voluminous and intrude over a period of c. 30 Ma, between c. 520 and 490 Ma. They are alkaline in composition and can be classified as A2-type. This indicates that they are probably formed by melting of a tonalitic to granodioritic lower crust in the cause of crustal thickening, or from underplated lower crust.

Thermobarometric constraints on the tectonothermal evolution of the East Humboldt Range metamorphic core complex, Nevada

The East Humboldt Range of Nevada provides a record of deep-crustal tectonic processes in a classic Cordilleran metamorphic core complex located in the hinterland of the Late Cretaceous to early Tertiary Sevier orogenic belt. New constraints reported here on the metamorphic history of this terrane suggest an overall clockwise pressure-temperature-time ( P-T-t ) path that began with deep tectonic burial and metamorphism at kyanite + staurolite + garnet grade before Late Cretaceous time (possibly in Late Jurassic time?). Subsequently, higher-temperature Late Cretaceous peak metamorphism overprinted this event, resulting in widespread partial melting and leucogranite injection contemporaneous with emplacement of a large-scale recumbent fold (the Winchell Lake nappe). A new 207Pb*/206Pb* date of 84.8 ± 2.8 Ma on syntectonic leucogranite from the hinge zone of this fold constrains the age of this major phase of tectonism. Metamorphism at this time probably reached the second sillimanite isograd, at least at deep structural levels, with peak P-T conditions of 800 °C and >9 kbar. High-grade conditions persisted during extensional tectonic denudation throughout much of Tertiary time. In conjunction with previously published work, the petrologic and thermobarometric results reported here for the northern East Humboldt Range delineate a steeply decompressional P-T trend that extends from ∼9 kbar and 800 °C to 5 kbar and 630 °C. In the light of decompressional reaction textures, microstructural evidence, and previously published thermochronometric results, we interpret this trend as a P-T-t path for Late Cretaceous to Oligocene time. At least 2 kbar of this decompression (equivalent to at least 7 km of denudation) occurred in Late Cretaceous to early Tertiary time. This interpretation supports the idea that tectonic exhumation of deep-crustal rocks in northeastern Nevada began during or immediately after the closing stages of the Sevier orogeny. Finally, the steepness of the proposed P-T-t path implies a thermal evolution from a colder to a much hotter geotherm, a circumstance that probably requires plastic thinning of the lower plate in addition to brittle attenuation and removal of the upper plate during Tertiary extension.

Structural and geochronological constraints on the role of partial melting during the formation of the Shuswap metamorphic core complex at the latitude of the Thor-Odin dome, British Columbia

At the latitude of the Thor-Odin dome, the Shuswap metamorphic core complex exposes a ~15 km thick structural section composed of an upper unit that preserved Mesozoic metamorphism, structures, and cooling ages, separated from the underlying high-grade rocks by low-angle detachment zones. Below the detachments, the core of the complex consists of an amphibolite-facies middle unit overlying a migmatitic lower unit exposed in the core of the Thor-Odin dome. Combined structural and super high resolution ion microprobe (SHRIMP) U-Pb geochronology studies indicate that the pervasive shallowly dipping foliation and east-west lineation developed in the presence of melt during Paleocene time. SHRIMP analyses of complexly zoned zircon grains suggest that the migmatites of the lower unit crystallized at ~56 Ma, and a syntectonic leucogranite at ~60 Ma. We suggest that leucogranite migrated upward from the migmatites through an array of dikes and sills that permeated the middle unit and ponded to form laccoliths spatially related to the detachment zones. The similarity in ages of inherited zircon cores in the two migmatite and the leucogranite samples suggests a genetic link consistent with the structural analysis. Following the crystallization of migmatites, the terrane cooled rapidly, as indicated by argon thermochronology. We propose that exhumation of the core of the Canadian Cordillera during the formation of the Shuswap metamorphic core complex occurred from ~60 to 56 Ma at a time when the crust was significantly partially molten. These structural and temporal relationships suggest a genetic link between mechanical weakening of the crust by partial melting, late-orogenic collapse, and exhumation of high-grade rocks in the hinterland of a thermally mature orogenic belt.

U-Pb geochronologic constraints on the age of thrusting, crustal extension, and peraluminous plutonism in the Little Rincon Mountains, southern Arizona

The Little Rincon thrust fault is a mylonitic shear zone that juxtaposes Middle Proterozoic Continental Granodiorite over metasedimentary rocks of Proterozoic and early Paleozoic age. This fault is structurally beneath the San Pedro detachment fault and associated ductile deformational fabrics, which formed during early Oligocene to early Miocene time. A syntectonic leucogranite sill within the Little Rincon shear zone yields a U-Pb concordia-intercept age 66{plus minus}10 Ma for zircon and a concordant age 51{plus minus}2 Ma for fractions composed of monazite and xenotime. This demonstrates that compressional deformation in the Catalina and Rincon mountains is generally coeval with Laramide thrust faults that extend at least from southeastern California to southeastern Arizona. A peraluminous granite pluton that truncates the shear zone but displays extension-related fabrics yields a lower-intercept age 24{plus minus}12 Ma for zircon and an age of 30{plus minus}6 Ma for monazite. This indicates that some peraluminous plutons in the region were emplaced during regional crustal extension.

A new tectonic model for the Cameroon Line, Central Africa

The Cameroon Line, a major geological feature in Central Africa, has been considered successively as a series of horsts and grabens, a continental rift and a mega-shear zone. It is marked out by about 60 anorogenic complexes and a dozen volcanic centres, all of which have alkaline affinity. Remote sensing allows us recognition of the main lineament trends: N70°, N-S, N135° and E-W, while autocorrelation analysis reveals a major fault zone striking N30° in western Cameroon and N15° in the northern region. A mega left-lateral shear zone is the model that best accounts for the fracture pattern and associated features such as linear and circular structures alignment of subvolcanic complexes, syntectonic leucogranites marking out older shear zones and vein dykes. The N70° Adamawa fault zone, a Pan-African fracture reworked during Albian-Aptian times, is the only shear zone of continental scale that could have initiated “en echelon” mega-tension gashes within the Cameroon Line during a Cainozoic left-lateral transcurrent movement.

Palaeozoic evolution of the Plateau d’Aigurande (NW Massif Central, France)

Summary The Plateau d’Aigurande represents the north-westernmost part of the French Massif Central. It is overlain by the sediments of the Paris Basin to the north and bounded by the La Marche shear zone to the south. Detailed mapping, mainly from the Creuse Valley region, indicates thrust and nappe tectonics. Beginning at the base of the sequence one can recognize: (1) the Fougères unit (schists of low to medium grade) intruded by syntectonic leucogranites; (2) the Eguzon unit (medium- to high-grade schists which are partly diaphtoretic in the lower part of the unit); (3) the Gargilesse unit (high-grade schists); and (4) the Migmatitic unit. The metasediments of the Fougères, Eguzon and Gargilesse units were originally coarse grained clastics. They are interbanded with magmatic units (orthogneisses and amphibolites). The migmatites are derived from greywacke-type rocks intruded by magmatic rocks (orthogneisses and amphibolites). Following a high-pressure (Silurian?) tectonometamorphic event, the piling up of tectonostratigraphic units occurred during two periods of deformation. The main one, probably of late Caledonian (Acadian?) age, was synchronous with or slightly after the climax of metamorphism. The second, of Westphalian age, was accompanied by the emplacement of leucogranitic magma and by retrograde metamorphism. The shear-sense appears to have been from SW to NE during the Acadian phase and from NW to SE during the Westphalian. Structural and lithological studies suggest that the three lowermost units may have been derived from the same palaeogeographic domain while the migmatites have a distinctly different history.