Population-wide garnet growth zoning revealed by LA-ICP-MS mapping: implications for trace element equilibration and syn-kinematic deformation during crystallisation
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40 Ar/ 39 Ar dates from eight muscovite separates and two whole-rock samples of slate, siltstone, and sandstone provide detrital, metamorphic, and post-metamorphic overprinting ages for the low-grade metamorphic rocks of the Meguma Supergroup in the Mahone Bay region of southern Nova Scotia. Variably reset detrital muscovite ages of 600-550 Ma agree with previously obtained U-Pb dates from detrital zircon. Separates of foliation-parallel muscovite yield precise 40 Ar/ 39 Ar ages of 395-388 Ma. Petrographic and metamorphic data indicate that these are metamorphic mica growth ages. The range overlaps with the younger limit of previously reported whole-rock ages (410-380 Ma), but is more tightly constrained. An apparent inconsistency between previous 40 Ar/ 39 Ar whole-rock ages as old as 410 Ma and regional deformation that affected sedimentary rocks as young as 385 Ma has been resolved by the younger ages reported here and by recent adjustments to the early Paleozoic time scale which suggest that the Lochkovian to Emsian Torbrook Group was deposited between 419 and 394 Ma. A muscovite plateau age of 376 Ma from one sample may be related to late-stage flexural slip deformation in the Ovens gold district.
Muscovite
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Geochronology
Siltstone
Phyllite
Devonian
Dalradian
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Abstract The ages obtained by the 40 Ar– 39 Ar encapsulation technique (retention and total gas ages) on <2 μm fractions of five metapelites from the Eastern Andean Metamorphic Complex and two from the Chonos Metamorphic Complex allow discussion of the latest recorded metamorphic event in each zone. The Kübler Index (KI) of illite/muscovite (principal component of the metapelites) varies between 0.15° and 0.45° Δ°2θ, indicating regional variation from diagenetic to epizonal metamorphic grade. The 40 Ar– 39 Ar encapsulation analyses reveal 39 Ar loss varying between 21 and 25%, which shows a limited positive correlation with KI values. The obtained retention and total gas metapelite ages reflect distinct metamorphic conditions. Retention ages most probably indicate burial or regional metamorphic events without plutonic influence in the southern Eastern Andean Metamorphic Complex. Total gas ages reflect contact ages for metapelites close to intrusions in the northern and southern Eastern Andean Metamorphic Complex and in the Chonos Metamorphic Complex. The thermal overprinting of metapelites occurred in Early Cretaceous times at 130 Ma and 145 Ma and is related to the contact metamorphism of an emplacement pulse of the North Patagonian Batholith. Total gas metapelite ages obtained from the western belt of the Chonos Metamorphic Complex suggest a thermal event related to a distinct pulse of the North Patagonian Batholith.
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Batholith
Muscovite
Metamorphic core complex
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Sequence (biology)
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The Rumbia Mountains, which in this study named Rumbia schist Complex is an east-west oriented, composed by a high-pressure/low-temperature, and a medium-pressure/low-temperature metamorphic rocks. Identified as mica schist, glauchopane schist, and green schist. Rumbia complex known as the location of gold deposits prospects discovered by local communities since 2007. The results of research showed that the metamorphic rocks are as hosts. There are two phase of gold mineralization that occurs in this area, namely: 1) Associated with tectonic deformation and metamorphic rocks exhumation, and 2) gold-related hydrothermal deposits. Radiometric age dating used 40Ar/39Ar geochronology, indicate that the first of gold mineralisation in the Rumbia Complex occurred ∼23 million years ago, and the second gold mineralisation were subsequently overprinting at 7 million years ago.
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The tectonometamorphic evolution of the marbles in the Lądek-Śnieżnik Metamorphic Unit, West Sudetes
Together with the adjacent rocks, the marbles of the Lądek-Śnieznik Metamorphic Unit (LSMU), West Sudetes, SW Poland underwent a polyphase structural evolution that occurred in metamorphic conditions changing from medium-grade to low-grade and in deformation regimes changing from ductile to semi-brittle. The structural evolution of the marbles began with E-W subhorizontal shortening resulting in the tilting of the Cal-Dol layering (which had generally originated as a pre-tectonic and pre-metamorphic feature) and its transposition to a steeply dipping metamorphic S1 foliation. The subsequent vertical flattening occurred at the temperature peak of regional metamorphism and produced the N-S trending tight, recumbent F2 folds. This event is also documented by the subhorizontal S2 axial-plane carbonate grain shape fabric and the parallel alignment of Phl-Ms±Tr±Czo in the marbles, and the S2 axial-plane schistosity in the adjacent mica schists. The temperature increase was associated with the progressive mineral sequence Phl → Tr → Di in the dolomite-bearing marbles, which probably initially equilibrated at low to moderate X(CO 2 ). Under peak temperatures, the observed arrangement of the metamorphic zonation of the Stronie formation developed, overprinting the folded planes. Subsequently, under retrogressive conditions, younger deformations were localised in the dynamically recrystallised shear zones that mostly reactivated the S2 planes and were associated with a late top-to-the-N (NE) directed tectonic transport. The D3 mylonitisation was associated with the elongation and size reduction of carbonate grains within the S-C' or S-C mylonites. It produced the S3 planes and the N-S trending L3 stretching lineation. Both groups of the tectonic structures and D2-established mineral isograds (Tr-in and Di-in) were together reoriented during the late compressional stages D4 and D5, related respectively to the SW-NE (WSW-ENE) and NW-SE (NNW-SSE) directed tectonic shortenings. This is visible in the large scale F4 folds, the diversity of the D2-and D3-related mineral assemblages, and the temperature estimations related to both tectonic stages, which indicate decreasing metamorphic conditions from ≥ 600°C in the SE to ≥ 490°C in the NE of the LSMU during D2, and from ca. 510°C to 430°C for the respective domains during D3. The incomplete pattern of the Di-in and Tr-in isograds, which still refers to the geometry of gneiss-schist boundary, confirms that the macrostructures of the LSMU mainly developed in tectonic event(s) following the temperature peak of metamorphism.
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Mylonite
Foliation (geology)
Lineation
Muscovite
Crenulation
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Abstract Raman spectroscopy of carbonaceous material (RSCM) thermometry is an effective method to determine the thermal structure of metasedimentary domains with high spatial resolution. The mean value of numerous measurements in a single sample is used for temperature estimation. A potential complicating factor is the presence of detrital carbonaceous material (CM) that can introduce bias in the results. Detrital CM generally shows a high degree of crystallinity making it difficult to discriminate detrital and metamorphic CM in high-grade metamorphic domains. Application of the RSCM thermometry to 100 metamudstone samples from the Hongusan area of the Ryoke metamorphic belt reveals multimodal degrees of crystallinity of CM in many samples indicating the common presence of detrital CM grains. Application of a new procedure to filter out the effects of detrital CM reveals a thermal structure that can be explained as the result of the overprinting of two distinct thermal regimes related to regional and subsequent contact metamorphism. The ability to recognize the presence of detrital CM and measure its crystallinity can be used to infer the metamorphic grade present in the hinterland from which the detrital material is derived. A combination of this information on metamorphic temperature with geochemical and radiometric age data reported in the literature indicates that the detrital CM was derived from basement rocks of the Korean Peninsula.
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