Carl Ehlers

The roots of the Dabieshan ultrahigh-pressure metamorphic terrane: constraints from geochemistry and Nd–Sr isotope systematics

The Dabieshan area between the Sino‐Korean and Yangtze cratons in east-central China has become the focus of much recent attention because of the discovery of abundant coesite and rare micro-diamond inclusions in both eclogites and their enclosing country rocks. The Dabieshan metamorphic complex, previously regarded as Archean continental basement of the Yangtze craton, is mainly made up of Precambrian felsic orthogneiss, amphibolite, and migmatitic gneiss with minor eclogite, granulite, ultramafic rock and marble. Our geochemical analyses and Nd‐Sr isotope data show that most Dabieshan orthogneisses are distinctly different from the nearby Kongling gneisses of the Yangtze basement, which are Archean high-Al TTG rocks with an average Nd model age of 3.39 0.2 Ga with volcanic arc granitic affinity. The protoliths of the Dabieshan orthogneisses are diverse, and three types of rocks are distinguished: (1) the majority of the felsic gneisses in the eclogite units display geochemical signatures of post-Archean granites and may have resulted from Neoproterozoic magmatism in a rift environment; (2) some of the felsic gneisses in the eclogite units have an affinity with Kongling gneiss, and were presumably derived from the Yangtze basement by tectonic extrusion during Mesozoic exhumation of the ultrahigh-pressure (UHP) metamorphic rocks; and (3) the felsic gneisses of the dome region show geochemical signatures of Archean granitoids and Nd model ages between 3.1 and 1.0 Ga, attributed to mixing between Neoproterozoic mantle-derived material and the Archean Kongling gneisses. Numerical modeling shows that mixing between mantle-derived melts and Kongling gneiss can account for the Nd‐Sr isotopic variation of Mesozoic mafic monzodiorites in the UHP eclogite unit, implying that the Kongling complex was extended beneath the Dabieshan terrane possibly during early Mesozoic continental collision. We suggest that the dome region was originally part of the Yangtze craton, and was separated from it by Neoproterozoic rifting. The orogen was later significantly modified, especially by Jurassic‐Cretaceous migmatization and magmatism.

The late Svecofennian granite-migmatite zone of southern Finland—a belt of transpressive deformation and granite emplacement

Abstract The late Svecofennian granite-migmatite (LSGM) zone in southwestern Finland is a ∼ 100 km wide and 500 km long belt transecting the southern Svecofennides from WSW to ENE. It was formed in an area of thin pillow lavas, volcaniclastic sediments and limestones. The area is interpreted as having been an early basin of crustal extension which was the locus of an inherited zone of weakness in the Proterozoic crust. Early recumbent folding was followed by crustal thickening and intrusions of ∼ 1.89-1.88 Ga old plutonics. The LSGM-zone is characterized by 1.84-1.83 Ga old rhomboidal sheets of late Svecofennian microcline granite and is bounded by ductile shears. Amongst the two major phases of deformation defined in the LSGM-zone, the earlier one (D1) affected only the supracrustals and the 1.89-1.88 Ga old early plutonics. In contrast, the later phase (D2) also deformed the late Svecofennian migmatites and granites. D1 represents a complex and long-lasting deformation event which included overturning and thrusting of the Svecofennian strata. D2 comprised ENE-WSW directed drag accompanied by NNW-SSE compression. The Svecofennian crust was thickened further and anatectic microcline granites intruded along thrusts. The rhomboidal outline of the late Svecofennian granite sheets indicates a sense of movement in agreement with measured dextral strike-slip in the shears delimiting the LSGM-zone. Imbricated feldspar megacrysts in the granites indicate thrusting towards the west during the stage of granitic magmatism. The gently dipping early Svecofennian gneisses and the late granite sheets were folded into upright F2 folds with gently plunging axes. Locally, the F2 axial surfaces were intruded by late Svecofennian granite mobilisates.

A post-collisional magmatic plumbing system: Mesozoic granitoid plutons from the Dabieshan high-pressure and ultrahigh-pressure metamorphic zone, east-central China

Three groups of Mesozoic shoshonitic or high-K calc-alkaline intrusive rocks are identified in Dabieshan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic zone, east-central China and they are related to: (I) slab breakoff; (II) magmatic underplating; and (III) doming. Group-I, the slab breakoff-type, consists of late Triassic (∼210 Ma) mafic monzodiorites. It has moderate to high Sr, and low Rb and Ba abundances, and moderate light rare earth element (LREE)/heavy rare earth element (HREE) and K/Rb ratios. Group-II, the underplating-type, consists mainly of middle Jurassic–early Cretaceous (160–120 Ma) hornblende quartz monzonitic, biotite monzogranitic, and syenogranitic plutons, characterized by relatively high LREE/HREE and K/Rb ratios, and by a large range in concentration of Sr and Ba, coupled with much smaller range in Rb. Group-III, the doming-type, is represented by Cretaceous (125–95 Ma) granitic stocks and granitic porphyry. Compared with group-II, it has high Rb, Y and HREE abundances, low Sr and Ba abundances and low LREE/HREE and K/Rb ratios. All groups have similar Nd and Sr isotopic compositions. Among the three groups, post-collisional granitoid magmatism (group-II) with ages of 160 to 120 Ma, post-dating the HP and UHP metamorphism at 245 to 220 Ma, is the most abundant in the Dabieshan area. The post-collisional granitoid plutons were initially emplaced at different levels ranging from mid-crust to near-surface. This study shows that the whole-rock chemistry of the granitoids vary systematically with crystallization pressures. For example, K2O, normative Or, Rb and Zr show the strongest increase with decreasing pressure, whereas Ba, Nb, Nd, Yb, MnO, and normative An decrease upward in the Dabie Block. It is suggested that ascent of differentiated, buoyant liquids, combined with fractionation paired with assimilation (AFC), is responsible for the vertical variation. Geological, geochemical and petrological data indicate that group-I could have been generated by partial melting of enriched subcontinental lithosphere mantle due to slab breakoff. Group-II rocks could have been produced mainly from crustal assimilation/melting and fractional crystallization of mantle-derived magmas, whereas group-III magma could have derived from anatexis of the Dabie complex and was highly evolved in a hot doming setting. The late Triassic-early Jurassic slab breakoff may be responsible for the exhumation of UHP rocks through the mantle. The voluminous granitic emplacement together with an episode of rapid denudation suggests that magmatic underplating and inflation could have played a role in the Middle Jurassic–Early Cretaceous rapid exhumation of Dabieshan.

The link between migmatites and S-type granites in the Turku area, southern Finland ☆

Abstract Metamorphism and compositions of metasedimentary migmatites and related small-scale granites are investigated in the Turku area (SW Finland). Phase assemblages and mineral compositions suggest peak P – T -conditions of ∼800 °C and 6 kbar. The portion of melt produced in the migmatites by dehydration melting is estimated to be ∼20 wt.%. The partial melt is K 2 O-rich and water undersaturated (∼4 wt.% H 2 O). The leucosomes and small-scale granites have almost the same proportions of felsic minerals, but the mafic minerals (Grt, Crd, Bt) are enriched in the granites along with the contents of TiO 2 , FeO tot , MgO and HFSE. The leucosomes have low total REE values and strongly positive Eu-anomalies. The REE patterns of the granites are characterized by relatively high LREE, low HREE and distinct negative Eu-anomalies. The distribution of major elements and trace element signatures suggests that leucosomes and granites are related as cumulates and evolved residual melts, respectively. The decimeter to meter-scale leucosomes are interpreted to be the product of crystal fractionation, which occurred during near-isobaric cooling, and the granites are the (crystallized) residual melts which could move further on and form larger bodies within and outside of the migmatite complex.

Exploration for dimensional stone : implications and examples from the Precambrian of southern Finland

Dimension stone is a natural rock that must fulfil high qualitative standards defined by both geology-based factors and non-geological factors. The stone itself (appearance/soundness) and the market demand are the two most important aspects in the quality assessment. The process of geological dimension stone exploration is a systematic and stepwise procedure, including individual steps of desk study, field mapping, detailed examination, geo-radar survey, and core drilling. The location of all economically feasible dimension stone deposits is strictly controlled by geological factors. Knowledge of these factors is fundamental in identifying new sites with potential for dimension stone. In this work the geological constraint on the occurrence of dimension stone in three areas, in the Precambrian of southern Finland, has been investigated. The deposits are localized to part of an area in which different geological features in combination have produced rock of good dimension stone quality. The study shows that, for example, vertical movements in the crust, magmatic evolution of an intrusion, and metamorphic grade define the location of rocks suitable for dimension stone.

Stratigraphy and geochemistry in the proterozoic mafic volcanic rocks of the Nagu-Korpo area, SW Finland

The volcanic rocks of the Nagu-Korpo area in SW Finland form a thin (< 1000 m) formation that is folded into narrow synforms separated by migmatite-filled antiforms. In Nagu the volcanic formation comprises three units: a lower unit of subvolcanic banded gabbro sills, a middle unit of amphibolitic volcanic rocks and an upper unit consisting of a thin ultramafic volcanic rock layer. The volcanic formation is interlayered within a thicker sequence of metasedimentary gneisses. Geochemically the metavolcanic rocks are tholeiitic and plots based on Ti, Y, Zr and other trace elements show that they resemble recent within-plate lavas. MORB- and mantle-normalised trace element plots show patterns strongly differing from those of recent arc lavas, indicating a tectonic environment of (plate boundary related?) initial rifting. The upper ultramafic lavas are enriched in LREE relative to the lavas of the much thicker amphibolitic middle unit below. A model for the initial formation of the volcanic rocks of the Svecofennian fold belt must acknowledge both the episode of initial rifting and the later evolution of an island arc-like environment.

Structural observations on the Uusikaupunki trondhjemite sheet, SW Finland

Abstract The Uusikaupunki trondhjemite is located in SW Finland within high-grade Palaeoproterozoic Svecofennian supracrustal rocks. Three phases of deformation are identified in the supracrustal rocks. The D1 deformation produced a schistosity defined by biotite. The D2 deformation is characterized by recumbent or reclined folds with NW vergences and it is accompanied by thrusting towards the NW. The N–S compression and the E–W shear motion define a transpressional tectonic regime during the D3 deformation. The D3 is associated with F3 folds overturned towards the N. It is suggested that the Uusikaupunki trondhjemite intruded as gently dipping sheets before or during the D2 deformation.

Early Proterozoic Svecofennian volcanism and associated plutonism in Enklinge, SW Finland

Abstract The c. 1.9 Ga old Enklinge volcano-plutonic centre is part of the Svecofennian province of SW Finland. The rocks of the area consist of an early subaquatic extrusion of mafic/intermediate to rhyolitic volcanics underlain by marbles, graywackes and quartz-feldspar-rich volcanoclastic sediments. The volcanic sequence is deformed and syntectonically intruded by a biotite tonalite accompanied and mixed with coeval basaltic dykes. Dacitic quartz porphyry dykes and small breccia pipes occur along the margins of the tonalite intrusion. Contrary to the Bergslagen area in Sweden no large co-magmatic granites can be found in the volcano-plutonic complex and the rhyolites are much less voluminous and only slightly mineralized. The rocks of the Enklinge area are strongly deformed and the tectonic environment differs strongly from that of the almost nonorogenic and more terrestrial Bergslagen area W. of Enklinge.

Homogenous deformation in precambrian supracrustal rocks of Kumlinge area, southwest Finland

Abstract Supracrustal rocks form a trough-like prolonged syncline surrounded by granitic rocks. They consist of quartz-feldspar schists (leptites) overlain by amphibolites. The stratigraphic succession in the syncline is as follows: undermost lie leptites followed by amphibolitic pillow lavas, which are gradually superseded by pillow breccias and agglomerates. A small area of polygonally jointed amphibolites lies above the pillow breccias. The succession gives the impression of volcanic activity in a gradually declining depth of water. The pillow lavas and agglomerates are commonly flattened or elongated and only locally undeformed. Elongation-type deformation is most pronounced in the agglomerates of the central parts of the syncline, where it now reaches its maximum breadth. The axial ratios of the elongated agglomerate fragments are approx. 2/3/75. Around the margins of the syncline the pillow lavas are strongly flattened. The most flattened pillows have axial ratios of approx 1/60/60. The flattening deformation in the adjacent leptitic rocks can be estimated from boudinage and folding in numerous pegmatite dikes. The estimated deformation in the leptites agrees reasonably well with the observed deformation in the adjacent pillow lavas. A gravitative sinking of the dense amphibolites into the underlying granitic material could possibly account for variations in the type of deformation within the syncline.

A Rb-Sr whole-rock isochron of 1.5 Ga in early Proterozoic pillow lavas in SW Finland

Abstract Svecofennian pillowed metalavas from SW Finland give Rb-Sr whole-rock ages close to 1.5 Ga and an initial strontium isotope ratio around 0.705. The low age is synchronous with the ages of two large rapakivi massives north and west of the pillow lavas, and it is suggested that it dates the thermal halo around the massives. The primitive and uniform initial strontium isotope ratios for the chemically slightly different tholeiitic pillow lavas show that they are partial melts of similar source rocks and that their different trace element composition is not due to contamination with older crustal material.