Low-pressure Metamorphic Belts Research Articles

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.

Late Palaeozoic 40Ar/39Ar ages of the HP-LT metamorphic rocks from the Kekesu Valley, Chinese southwestern Tianshan: new constraints on exhumation tectonics

abstractAlthough numerous ages have been obtained for the Chinese southwestern Tianshan high pressure/ultrahigh pressure-low temperature (HP/UHP-LT) metamorphic belt in the past two decades, its exhumation history is still controversial. The poor age constraint was related to the appealing low metamorphic temperatures and excess Ar commonly present under HP/UHP conditions. This study aims to provide new age constraints on the orogen’s exhumation by obtaining 40Ar/39Ar mica ages using the conventional step-heating technique, with emphasis on the avoidance of excess Ar contamination. From a cross section along the Kekesu Valley, four samples, three from the HP-LT metamorphic belt (TK050, TK051, and TK081) and one from the southern margin of the low pressure metamorphic belt (TK097), were selected for 40Ar/39Ar dating. Phengites from garnet glaucophane schist TK050 and the surrounding rock garnet phengite schist TK051 yield comparable plateau ages of 321.4 ± 1.6 and 318.6 ± 1.6 Ma, respectively, while epidote mica schist TK081 gives a younger plateau age of 293.3 ± 1.5 Ma. Considering the chemical compositions of phengites, mineral assemblages, and microstructures in the thin slices, we suppose that the former represents the time the HP rocks retrograded from the peak stage (eclogite facies) to the (epidote)-blueschist facies, whereas the latter reflects greenschist facies overprinting. Biotite and muscovite from two-mica quartzite TK097 give similar plateau ages of 253.0 ± 1.3 and 247.1 ± 1.2 Ma, interpreted to date movement on the post collisional transcrustal South Nalati ductile shear zone. By combining our new ages with published data, a two-stage exhumation model is suggested for the Chinese southwestern Tianshan HP/UHP-LT metamorphic belt: initial fast exhumation to a depth of about 30–35 km by ~320 Ma was followed by relatively slow (~1 mm year–1) uplift to ~10 km by ~293 Ma.

High radiogenic heat–producing granites and metamorphism—An example from the western Mount Isa inlier, Australia: Comment and Reply

[Extract] We welcome the paper of McLaren et al. (1999) because it makes an important contribution to our understanding of the source of thermal anomalies in low-pressure metamorphic belts. However, we argue that high radiogenic heat–producing granites such as the Sybella batholith, although locally important in the overall thermal budget, were not the primary cause of low-pressure metamorphism in the Mount Isa inlier, and that synmetamorphic intrusions were a significant additional factor.

Upper- and middle-crustal response to delamination: An example from the Lachlan fold belt, eastern Australia

A delamination model applied to the Lachlan fold belt in eastern Australia shows that lithospheric delamination began in the Early Silurian (∼430 Ma), after crustal thickening in the Wagga-Omeo zone. The delamination induced isostatic uplift farther east to produce localized, transient extensional basins and coeval silicic magmas, which formed by partial melting of the lower crust during asthenospheric upwelling. However, the voluminous granitoid intrusions were emplaced during and after compression, reflecting reestablishment of horizontal compression associated with plate convergence. Continued sinking of the slab produced an east. ward-widening orogenic belt where the granite batholiths and associated thermally softened zones of deformation tracked the direction of delamination. Once the slab was completely detached at ∼400 Ma, the compressive stresses were transmitted across the belt, and delamination recommenced about 1000 km westward against the cratonic margin; the process was repeated at 400-370 Ma. Within 60 m.y., the two sinking slabs had converted a 1700-2000-km-wide continental margin into a 700-km-wide orogenic belt. Important effects of delamination on the upper and middle crust are (1) production of regional-scale, low-pressure metamorphic belts of limited crustal thickness; (2) widespread voluminous silicic magmatism; (3) lack of deep-crustal exposures even with high degrees of crustal shortening; and (4) partial replacement of the lower crust by a mafic underplate that cooled completely after deformation ceased.

Palaeozoic arc growth, deformation and migration across the Lachlan Fold Belt, southeastern Australia

The Palaeozoic Lachlan Fold Belt (LFB) is a low-pressure metamorphic belt characterized by greenschist-facies rocks and infolded volcanic sequences, which indicate that the belt has not undergone substantial uplift at any stage. Porphyroblast/ matrix relationships in one of the highest-grade metamorphic zones of the LFB, the Omeo Complex, indicate that the thermal peak was reached before, or was synchronous with, the earliest deformation, similar to that in high-grade metamorphic zones of the northern Arunta Inlier, central Australia. In both areas, the metamorphism is of low- P, high- T type, centred on major granite intrusions as regional aureoles, and characterized by anti-clockwise P- T- t-paths. However, in the outer (lower-grade) parts of the aureoles, peak metamorphic conditions post-date the earliest deformation. This diachronous relationship indicates that (1) outward migration of a thermal front was centred on major granitoid intrusions (batholiths), (2) deformation propagated outward more rapidly than migration of the thermal front, and (3) plastic and ductile deformation began only after heating around the batholiths. Thus, orogeny was initiated after, and localized by, heat-focusing in the mid-crust associated with batholith emplacement. Therefore, the early deformation is a result of thermal softening: it is typically subhorizontal ductile shear at mid-crustal levels, but is characterized by upright to inclined folds at upper crustal levels. In the latter environment, the typically discordant “contact-aureole” plutons are often interpreted as post-tectonic granitoids, but they were part of the ongoing mid-crustal thermal perturbation that induced regional greenschist-facies metamorphism, which overprinted the early-formed, upright folds as it migrated to upper crustal levels. Orogeny migrated generally eastward through the LFB. It began with the Early Silurian Benambran Orogeny, centred on the Wagga Metamorphic Belt (WMB), and terminated with the Carboniferous Kanimblan Orogeny, which was most intense in the Hill End Trough. Eastward migration of orogeny across central Victoria, part of the West LFB, began in the Ballarat-Bendigo Zone during the Early Devonian and terminated in the Melbourne Zone in the Middle Devonian. Orogeny was centred on the meridional batholiths of the LFB, each of which are considered to represent the transitory axis of an ancient magmatic arc. Stepwise, but generally eastward, arc migration caused the eastward migration of orogeny. A tectonic model, based on the modern southwest Pacific arc system, can be applied to the LFB. Following Cambrian intra-oceanic arc growth associated with west-dipping subduction, an Ordovician marginal sea developed, partly on stretched Cambrian crust, and was flooded by an extensive turbiditic wedge. Closure of the eastern part, the East LFB, began in the Early Silurian after a magmatic arc developed over the WMB, possibly associated with an east-dipping subduction zone. After re-establishment of west-dipping subduction in the Middle Silurian, absolute-motion retreat of the upper (Australian) plate, caused by oblique plate convergence, resulted in dextral transtension and the generation of Siluro-Devonian inter-arc and back-arc basins. Transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard (eastward) arc system. Periodic compression within the evolving arc and back-arc system resulted in a series of east-younging, east-verging, linear, Siluro-Devonian fold-and-thrust belts, which were localized adjacent to elongate batholiths that represent the relict arc systems. Local westward jumps in deformation and plutonism occurred. The most significant is the $ ̃ 600 km westward jump into the West LFB, caused by closure of the remainder of the passive Ordovician back-arc basin (Melbourne and Bendigo-Ballarat zones). West-dipping subduction beneath the basin produced a low-pressure fold-thrust belt similar to the East LFB, though the granitoids have a more primitive isotopic character, indicating closure of thin, dominantly oceanic crust in the Early to Middle Devonian. Localized deformation, induced by thermal softening of the crust within magmatic arcs that have developed on earlier passive margins, is likely to produce anticlockwise P-7- t-paths and may be a typical feature of southwest Pacific-style tectonism.

Thermal development of low‐pressure metamorphic belts: Results from two‐dimensional numerical models

We have used two‐dimensional numerical models and analytical solutions to heat flow equations to investigate the mechanisms and the spatial and temporal development of regionally extensive low‐pressure facies‐series metamorphism (LPM). Two‐dimensional models are necessary for describing the evolution of isotherms in the crust where lateral heat flow and local time‐dependent heat sources are important. The models demonstrate that felsic intrusions can produce regionally extensive LPM where their abundance through time in the upper crust exceeds approximately 50%. Evenly spaced intrusions elevate vertical metamorphic gradients in regions between intrusions by ∼5° to 10°C km−1 over background values at ∼40% abundance and by ∼15° to >30°C km−1 at ∼70% abundance. Other mechanisms, such as intrusion of mafic magmas in the lower crust, crustal extension, or aqueous fluid advection, cannot independently produce maximum temperatures in such regions. If coeval with intrusion, these other processes will contribute to maximum temperatures; however, the distribution of isotherms and the timing of metamorphism in the upper crust are governed by the shallow intrusions. Intrusion width, abundance, and composition, and timing of emplacement of nearby intrusions have the dominant influences on maximum temperatures between intrusions. For 10‐km‐wide intrusions, abundances necessary for production of regionally extensive LPM are ∼15% less for gabbros than for granites and ∼20% less when intrusions are emplaced simultaneously than when complete cooling occurs between intrusive events. Intrusion shape, height, convection (magmatic and hydrothermal), and recharge (open system magma chambers) have a significantly less effect on thermal evolution. The total area affected by intrusions depends on their final distribution, not the magmatic flux. In the development of low‐pressure metamorphic terranes with abundant intrusions, temperatures in the upper crust are significantly elevated only near recently emplaced intrusions. Consequently, the final distribution of metamorphic grades has little resemblance to the distribution of isotherms at any time—a fundamental difference with LPM in regions lacking abundant shallow intrusions, where metamorphism likely occurs concurrently over large regions. This model of the development of widespread LPM by the juxtaposition through time of local, short‐lived thermal maxima should be considered when interpreting temperature‐dependent phenomena (for example, deformation) in terranes with abundant intrusions.

Magmatism and the development of low-pressure metamorphic belts: Implications from the western United States and thermal modeling

Research Article| August 01, 1989 Magmatism and the development of low-pressure metamorphic belts: Implications from the western United States and thermal modeling MARK D. BARTON; MARK D. BARTON 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90024 Search for other works by this author on: GSW Google Scholar R. BROOKS HANSON R. BROOKS HANSON 2Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1989) 101 (8): 1051–1065. https://doi.org/10.1130/0016-7606(1989)101<1051:MATDOL>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation MARK D. BARTON, R. BROOKS HANSON; Magmatism and the development of low-pressure metamorphic belts: Implications from the western United States and thermal modeling. GSA Bulletin 1989;; 101 (8): 1051–1065. doi: https://doi.org/10.1130/0016-7606(1989)101<1051:MATDOL>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Two-dimensional numerical modeling and geological and geophysical constraints from ancient and modern magmatic arcs demonstrate that magmatic heat advection is sufficient to produce low-pressure metamorphic belts in many areas, and that it is apparently necessary in some areas. In the western United States and other areas, regionally extensive low-pressure facies-series metamorphism (LPM) occurs where intrusions form >∼50% of the upper crust. Numerical models indicate that coalescence of thermal aureoles from multiple felsic intrusions can produce regionally extensive LPM where the abundance of intrusions exceeds ∼50%. This effect does not depend strongly on the rate of emplacement; LPM results even with complete cooling between intrusions. Models with geologically reasonable emplacement rates show that in an active magmatic arc, temperatures are near metamorphic maxima for only a small fraction of the time. Arc magmatism cannot sustain widespread thermal gradients of the magnitude indicated by the final distribution of LPM, a result consistent with heat-flow data in active arcs. Low-pressure metamorphic belts can thus develop through numerous local, short-lived metamorphic events while most of the crust remains considerably cooler. Metamorphic maxima largely depend on the biggest nearby intrusion; emplacement rates and other heat sources affect mainly the magnitude, not the distribution of metamorphism.A simulation based on the distribution and U-Pb geochronometry of the composite Sierra Nevada batholith compares favorably with the observed distribution of metamorphic grades and temperature histories; coeval medium-pressure facies-series metamorphism to the east of the batholith requires a different mechanism. Predicted surface heat flow is qualitatively similar in magnitude and distribution to that measured in modern arcs. Sequential intrusions can produce brief (<1 m.y.) high-grade events after several million years of lower-grade metamorphic conditions, providing a mechanism for hornfelsic textures to overprint schistose textures formed during the same event. Alternative mechanisms for LPM include (1) rapid uplift during extension or following crustal thickening and (2) deep heat sources (including magmas). These mechanisms are inconsistent with the geologic record in areas such as the western United States, where large near-surface gradients, lack of requisite major (>10 km) uplift, and evidence for moderate over-all paleothermal gradients require high-level advection of heat. This model of relatively cool crust punctuated by local, short-lived thermal events differs significantly from the image of a broad, uniform thermal maximum that might be drawn from the spatial distribution of LPM; it has implications for crustal deformation igneous petrogenesis, and the interpretation of high metamorphic gradients in Archean terranes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Northern margin of the Indian Plate—some litho-tectonic constraints

The Indus-Tsangpo Suture Zone represents crustal expression of a subduction zone in the Himalaya. All along its extension in Kohistan, Ladakh and Tibet, the suture zone from south to north is constituted of litho-tectonic assemblages referable to trench zone, fore-arc basin, island arc and back-arc basin. These assemblages lie between the platform sediments deposited at the northern margin of Indian and southern margin of Tibet. The trench assemblage consisting of ophiolitic assemblages exhibits high pressure glaucophanitic metamorphism. The flysch sequence of the Indus Group represents fore-arc sediments deposited in front of the Ladakh magmatic arc comprising Dras volcanics, the Ladakh plutonic complex and the Khardung volcanics. To the north of the magmatic arc, the back-arc basin assemblage represented by the Shyok Group consists of acid and basic volcanics and volcanoclastics intruded by granitic plutons. The rocks of the arc and back-arc region exhibit low pressure, high temperature metamorphism. The high pressure and low pressure metamorphic belts lying to the south and north respectively of the magmatic arc constitute paired metamorphic belts. Detailed analysis of tectonic and lithological characters of various litho-tectonic units indicate that these developed at the northern margin of the Indian Plate during the Mesozoic and Early Tertiary. The Miocene-Pliocene molassic rocks overlying various litho-units unconformably were deposited after the collision of the Indian and Tibetan plates.

Role of plutonism in low-pressure metamorphic belt formation

Wickham and Oxburgh1 recently proposed that low-pressure/high-temperature (low-P/high-T) metamorphism in the eastern Pyrenees, and possibly all low-P/high-T metamorphic belts, resulted from anomalously high mantle heat flow brought about by rifting. Their model is largely constrained by the presence of nearby synmetamorphic rift-related sedimentary rocks and the interpretation that the migmatites and granites are the product of in situ melting in the presence of an anomalously steep geotherm. Here we present an alternative model, in which low-P/high-T metamorphism (pro-grade reactions at pressures near or below the Al2SiO5 triple point) results from contact effects near sill-like igneous intrusions at intermediate crustal levels. Low-P/high-T conditions can be achieved through this process in regions of continent–continent collision with normal mantle heat flux as well as in zones of extension. Our model is based on studies of the low-P/high-T metamorphic terrane in the New England Appalachians.

Paleogene metamorphism of an accretionary flysch terrane, eastern Gulf of Alaska

Reconnaissance field studies have identified a regional metamorphic complex, at least 200 km long and 25 to 50 km wide, in the eastern Chugach Mountains, southern Alaska. The metamorphic complex was developed within the Cretaceous Valdez Group, a flysch sequence that was accreted to the continental margin during the Late Cretaceous or early Tertiary. The metamorphic complex shows the progressive development from subgreenschist and green-schist facies slate, phyllite, and semischist that is regionally characteristic of the Valdez Group, to biotite-plagioclase-quartz schist (± muscovite, cordierite, andalusite, staurolite, garnet, and sillimanite) in an intermediate zone, and amphibolite facies muscovite-biotite-plagioclase-quartz schist and gneiss (± sillimanite and garnet) in a higher-grade central zone. K-feldspar is characteristically absent in the amphibolite facies schists, but it is present in migmatitic rocks that are common to the central zone. The mineral assemblages and petrographic relations indicate that metamorphism took place under low-pressure intermediate-facies series conditions; partial melting took place at the highest grade of metamorphism. Melting produced granodioritic magmas in migma-titic rocks of the central zone that were emplaced as discrete plutons in both the intermediate zone and peripheral rocks of the complex. The timing of metamorphism as indicated by field relations and K-Ar data is about 50 to 65 m.y. ago and overlaps or is coeval with the emplacement, throughout the Gulf of Alaska region, of early Tertiary plutons that are thought to be anatectic in origin. The distribution of these plutons suggests that a high-temperature low-pressure metamorphic belt, about 1,800 km long, developed along the leading edge of the continental margin in late Paleocene to early Eocene time.

Paired and unpaired metamorphic belts

Paired metamorphic belts occur in many parts of the circum-Pacific regions. A pair is composed of two contrasting belts running parallel: a high-pressure metamorphic belt which probably formed beneath a trench zone, and a low-pressure metamorphic belt which probably formed beneath a volcanic chain in the adjacent island arc or continental margin. The former and the latter belt are accompanied by basic-ultrabasic rocks and by granitic-andesitic-rhyolitic rocks, respectively. The rapid descent of a thick, cold oceanic plate along a convergent plate juncture should create an unusually low geothermal gradient to cause high-pressure metamorphism. If the rate of plate descent is relatively slow, or if the descending oceanic lithosphere is too thin or too hot, the resultant geothermal gradient will not be low enough to cause high-pressure, but may cause medium-pressure metamorphism. In this case, the contrast between the two associated metamorphic belts will be obscure. The heat transfer by the rise of magmas and mantle materials appears to be a necessary condition for the formation of low-pressure metamorphic belts. Presumably, paired and unpaired (single) metamorphic belts form by the same mechanism, and an unpaired belt represents paired belts in which the contrast between the two belts is obscure, or in which one of the two belts is undeveloped or lost. Progress in experimental petrology enables us to estimate the pressure and temperature during metamorphism, and to know the relations between the conditions of partial melting and the composition of the resultant magmas. This sets limits to our ideas about the relevant tectonic processes in plate junctures.