Radioactive element distribution in the Archean granulite terrane of Jequié — Bahia,Brazil

The radioactive element distribution in the Archean granulite facies rocks of Jequié, Brazil, has been determined. The K/Rb ratios show a large spread with majority of the values falling within the normal field (K/Rb<300). The Th/U ratios fall into two groups, one group with values in the range of 1.7 to 2.6, whereas the other group show Th/U values greater than 7 (7 to 25). These distributions can be explained on the basis of the presence of accessory minerals apatite and zircon. The relatively high concentrations of the radioactive and other elements in Jequié granulites probably represents the relict primary geochemical character, little modified during metamorphism. The radiogenic heat production data for the area are calculated and using the measured regional heat flow value from nearby area an estimate of the relative thickness of the continental lithosphere is made using the model developed by Oxburgh (1981).     

Origin of granites in an Archean high-grade terrane,southern India

Archean deep-level granites in southern India are similar geochemically to young granites from continentalmargin arc systems. They exhibit light REE enriched patterns with variable, but chiefly positive Eu anomalies. This is in striking contrast to the negative Eu anomalies typical in high-level Archean granites. In addition, the deep-level granites are relatively enriched in Ba and Sr and depleted in total REE and high field strength elements (HFSE). One pluton, the Sankari granite, has unusually low contents of REE and HFSE. Most of the deep-level granites appear to represent cumulates with variable amounts of trapped liquid and of minor phases, resulting from fractional crystallization of a granitic parent. Such parental granitic magmas can be produced by batch melting of Archean tonalite at middle to lower crustal depths. The Sankari granite requires a tonalitic source depleted in REE and HFSE. Archean tonalites and tonalitic charnockites exhibit original igneous geochemical signatures and their average composition does not show a significant Eu anomaly. Hence, they cannot represent the positive Eu-anomaly complement to the negative Eu-anomaly, high-level granites. Our results suggest that Archean deep-level granites may represent this complement. Such granite may form in waterrich zones in the middle or lower crust and be produced in response to dehydration of the lower crust by a rising CO₂-rich fluid phase.     

Large-scale melt-depletion in granulite terranes: an example from the Archean Ashuanipi Subprovince of Quebec

This study uses field, petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane, and to determine what the grain‐ and outcrop‐scale melt‐flow paths were during the melt segregation process. The Ashuanipi subprovince, located in the north‐eastern Superior Province of Quebec, is a large (90 000 km²) metasedimentary terrane, in which > 85% of the metasediments are of metagreywacke composition, that was metamorphosed at mid‐crustal conditions (820–900 °C and 6–7 kbar) in a late Archean dextral, transpressive orogen. Decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase, together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction: biotite + quartz + plagioclase = melt + orthopyroxene + oxides. Using melt/orthopyroxene ratios for this reaction derived from experimental studies, the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol% partial melting. The metagreywackes are enriched in MgO, CaO and FeO_t and depleted in SiO₂, K₂O, Rb, Cs, and U, have lower Rb/Sr, higher Rb/Cs and Th/U ratios and positive Eu anomalies compared to their likely protolith. These compositions are modelled by the extraction of between 20 and 40 wt %, granitic melt from typical Archean low‐grade metagreywackes. A simple mass balance indicates that about 640 000 km³ of granitic melt was extracted from the depleted granulites. The distribution of relict melt at thin section‐ and outcrop‐scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary (porous) flow from the site of melting, across the layers and into bedding planes between adjacent layers. In other rocks pervasive grain boundary flow of melt occurred along the layers for a few, to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits, visible as the net‐like array of leucosomes in some outcrops. The leucosomes contain very little residual material (< 5% biotite + orthopyroxene) indicating that the melt fraction was well separated from the residuum left in situ as melt‐depleted granulite. Only 1–3 vol percentage melt remained in the melt‐depleted granulites, hence, the extraction of melt generated by biotite dehydration melting in these granulites, was virtually complete under conditions of natural melting and strain rates in a contractional orogen.     

Thermal gradients in the Sri Lankan granulite terrane: a garnet–biotite thermometric approach

Thermal zoning of the Highland Complex, Sri Lanka has been delineated using the Fe²⁺–Mg distribution coefficient between garnet and biotite from garnet–biotite gneiss samples collected with wide geographical distribution. In order to minimize the potential for retrograde Fe–Mg exchange and maximize the potential for retaining peak equilibrium K_D (garnet–biotite) and temperature, garnet and biotite included within feldspar and quartz without other mineral inclusions have been selected. The calculated results indicate four distinct temperature contours with K_D values varying from 1.84 to 6.38 and temperature varying from 996 to 591 °C. From the present results, it is possible to divide the Highland Complex into two major metamorphic zones: a high‐temperature area in the central region and a low‐temperature area in the south‐western and north‐eastern region. In conjunction with the metamorphic pressure variations estimated from the granulites of the Highland Complex in previous studies, it is shown that the high‐ and low‐temperature areas are complemented by a high‐pressure region towards the eastern side and a low‐pressure region towards the western side of this complex. This thermal dome is interpreted to be an artifact of the different crustal levels exhumed following Pan‐African metamorphism.     

Partial melting during tectonic exhumation of a granulite terrane: an example from the Larsemann Hills, East Antarctica

Anatectic migmatites in medium- to low-pressure granulite facies metasediments exposed in the Larsemann Hills, East Antarctica, contain leucosomes with abundant quartz and plagioclase and minor interstitial K-feldspar, and assemblages of garnet–cordierite–spinel–ilmenite–sillimanite. Qualitative modelling in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂, in conjunction with various P–T calculations indicate that the high-grade retrograde evolution of the terrane was dominated by decompression from peak conditions of c. 7 kbar at c. 800 °C to 4–5 kbar at c. 750 °C. Extensive partial melting during decompression involved the replacement of biotite by the assemblage cordierite–garnet–spinel within the leucosomes. These leucosomes represent the site of partial melt generation, the cordierite–garnet–spinel–ilmenite assemblage representing the solid products and excess reactants from the melting reaction. The extraction and accumulation of this decompression-generated melt led to the formation of syntectonic pegmatites and extensive granitic plutons. Leucosome development and terrane decompression proceeded during crustal transpression, synchronous with upper crustal extension, during a progressive Early Palaeozoic collisional event. Subsequent retrograde evolution was characterized by cooling, as indicated by the growth of biotite replacing spinel and garnet, thin mantles of cordierite replacing spinel and quartz within metapelites, and garnet replacing orthopyroxene and hornblende within metabasites. P–T calculations on late mylonites indicate lower grade conditions of formation of c. 3.5 kbar at c. 650 °C, consistent with the development of late cooling textures.     

Late Archean magmatic complexes of the Azov terrane,Ukrainian Shield: Geological setting,isotopic age,and sources of material

Geochemical, isotopic-geochemical, and geochronological information was obtained on magmatic rocks from the Saltychan anticlinorium in the Azov domain of the Ukrainian Shield. The rocks affiliate with the calc-alkaline series and a high-Mg series. The rocks of these series notably differ in concentrations of trace elements and REE and range from gabbro to granodiorite-quartz diorite in composition. The NORDSIM ionprobe U-Pb zircons ages of rocks belonging to the Obitochnen Complex and having both elevated and normal mg# correspond to 2908–2940 Ma. The Osipenkovskaya intrusion has an age of 2855 ± 19 Ma. The most alkaline North Obitochnen intrusion was emplaced in the Proterozoic, at 2074 ± 11 Ma. The age of the amphibolite metamorphism of the host gneisses is reliably dated at 3120–3000 Ma. The model Sm-Nd ages of the intrusive rocks do not exceed 3150 Ma. According to geochemical evidence, the parental melts of the magmatic rocks were derived from mantle domains variably enriched in lithophile elements. The results obtained by studying the Sm-Nd isotopic system corroborate the conclusion drawn from geochemical evidence that most of the melts were derived from the mildly enriched mantle, practically without involvement of ancient crustal material. The mantle became enriched in LREE at approximately 3000 Ma, which corresponds to the age of metamorphism of the supracrustal rocks. This process was separated from the derivation of the melts by a time span of 70–80 Ma. The relative age of the intrusive rocks and their variable composition can be most adequately explained by a contribution of heat and material from a plume to the derivation of the parental melts of these rocks.     

Conditions of Archean granulite metamorphism in the Godthab-Fiskenaesset region, southern West Greenland

The Nordlandet peninsula (Akia terrane) and the Tasiusarsuaq terrane in southern West Greenland were metamorphosed to granulite facies at 3.0 and 2.8 Ga respectively. Temperatures of metamorphism are estimated using magnetite + ilmenite, garnet + orthopyroxene, garnet + clinopyroxene and garnet + biotite thermometers. Barometry has been carried out in the two terranes using eight different garnet barometers. A uniform set of activity models for all minerals, including the garnet activity model of Newton et al. (1986), is applied to each barometer in order to permit comparison. Pressure estimates using the different barometers are generally quite consistent (±1.5 kbar). Use of the Newton et al. (1986) garnet activity data results in pressures similar to those obtained using other garnet activity models.
Peak metamorphic conditions on the Nordlandet peninsula are estimated to have been 800 ± 50°C, 7.9 ± 1.0 kbar. Values of log f _O2 are estimated to have been 1.1 to 2.0 above the quartz + magnetite + fayalite buffer from assemblages of magnetite + ilmenite and quartz + magnetite + ferrosilite. Peak metamorphic conditions in the Tasiusarsuaq terrane are estimated to have been 780 ± 50°C, 8.9 ± 1.0 kbar. Estimates of f _H2O and f _CO2 using biotite, amphibile, grossular + anorthite and grossular + scapolite equilibria are low in both terranes. These results suggest that granulite metamorphism was fluid absent in both terranes, and that the metamorphism in the Akia terrane and possibly also in the Tasiusarsuaq terrane was initiated by the injection of large volumes of magma into the lower crust.     

华北克拉通南缘下地壳麻粒岩地体的形成与演化

研究前寒武纪地体的构造-岩浆事件和变质作用过程是揭示早期陆壳的生长与演化等热点问题的重要途径。目前,尽管已经开展了大量的相关研究,但关于华北克拉通南缘前寒武纪变质地体的成因、构造背景仍存在争议,限定了我们对于华北克拉通南缘前寒武纪形成和演化的认识。本文根据前人研究成果,包括华北南缘太华杂岩的岩相学、地球化学、年代学以及变质P-T-t轨迹等,结合对鲁山地区太华杂岩的全岩主微量、长石矿物原位Pb同位素组成特征以及基性麻粒岩的年代学等研究成果,系统梳理并探讨了太华杂岩代表的新太古代-古元古代下地壳物质组成、岩石成因以及多阶段构造-岩浆事件。结果显示华北南缘记录了2.92~2.71Ga和2.53~2.45Ga两期重要的地壳生长事件,2.35~2.30Ga和2.19~2.06Ga的陆壳再造,以及1.96~1.84Ga古元古代的变质事件。再结合华北克拉通中部造山带中、北部的变质程度、变质P-T轨迹和变质年龄,我们认为华北南缘前寒武纪基底与中部造山带的中、北部相似,构造归属于中部造山带,发生了一个较长的俯冲碰撞造山过程(1.96~1.86Ga)。

     

Unravelling the tectonothermal evolution of reworked Archean granulite facies metapelites using in situ geochronology: an example from the Gawler Craton,Australia

The use of in situ geochronological techniques allows for direct age constraints to be placed on fabric development and the metamorphic evolution of polydeformed and reworked terranes. The Shoal Point region of the southern Gawler Craton consists of a series of reworked granulite facies metapelitic and metaigneous units which belong to the Late Archean Sleaford Complex. Structural evidence indicates three phases of fabric development with D₁ retained within boudins, D₂ consisting of a series of upright open to isoclinal folds producing an axial planar fabric and D₃ composed of a highly planar vertical high‐strain fabric which overprints the D₂ fabric. Th–U–total Pb EPMA monazite and garnet Sm–Nd geochronology constrain the D₁ event to the c. 2450 Ma Sleaford Orogeny, whereas the D₂ and D₃ events are constrained to the 1730–1690 Ma Kimban Orogeny. P–T pseudosections constrain the metamorphic conditions for the Sleafordian Orogeny to between 4.5 and 6 kbar and between 750 and 780 °C. Subsequent Kimban‐aged reworking reached peak metamorphic conditions of 8–9 kbar at 820–850 °C during the D₂ event, followed by high‐temperature decompression to metamorphic conditions <6 kbar and 790–850 °C associated with the development of the D₃ high‐strain fabric. The P–T–t evolution of the Shoal Point rocks reflects the transpressional exhumation of lower crustal rocks during the Kimban Orogeny and the development of a regional ‘flower structure’.     

Proterozoic highlights of the Canadian Shield: A review

Stockwell's structural divisions of the Canadian Shield, and his Precambrian time‐classification based on major tectono‐thermal events are reviewed. The three main divisions of Proterozoic time, the Aphebian, Helikian, and Hadrynian Eras, follow the Kenoran, Hudsonian and Grenvillian Orogenies, whose terminations are currently dated on the U‐Pb scale at about 2550, 1750, and 1000 m.y. before present, respectively.

Proterozoic supracrustal sequences are most commonly of shallow marine or continental origin, and high‐level intrusions predominate. ‘Classical’ geosynclinal cycles have been restricted to the Aphebian Era.

High‐grade Aphebian rocks in the mid‐Churchill Province evidently mark the peak of Hudsonian tectonism. Subsequently, the Shield became relatively stable except for the Grenville Province, and deposition and deformation resulted from epeirogenesis, block‐faulting and rifting, mainly in the western Shield. Proterozoic sedimentation and volcanism likewise diminished in Helikian time and terminated in early Hadrynian time, about 800 m.y. ago.

The history of the Grenville Orogen remains obscure. The continuity of major Archaean to Helikian rock units into the Grenville from adjacent provinces constrains proposals for a marginal plate boundary and convincing evidence for Proterozoic plate boundaries within the Grenville Province is lacking, palaeomagnetic evidence notwithstanding.     

Differential response of U-Pb systems in coexisting accessory minerals,Winnipeg River Subprovince,Canadian Shield: implications for Archean crustal growth and stabilization

The U-Pb isotopic systems of zircon, monazite, titanite and some apatite and the Pb isotopic composition of K-feldspar have been investigated in three areas of the Winnipeg River Subprovince (WRS) of the Superior Province, Canada, in order to define the timing of magmatic and metamorphic processes in this Archean gneissic-granitoid terrain.The new data together with published results define the following stages in the evolution of the WRS: (1) an extended period of early crustal growth punctuated by the episodic generation of tonalite. New ages include 3170+20/s-5 Ma, 2875+20/s-5 Ma and 2840+20/s-5 Ma for tonalitic gneisses at Cedar Lake, Kenora and Daniels Lake, respectively. (2) This early evolution was concluded by about 2760 Ma after emplacement of tonalite-granodiorite at Cliff Lake and was followed by a period of magmatic quiescence between about 2760 and 2710 Ma that contrasts with the intensive igneous activity characterizing the evolution of neighbouring greenstone belts. (3) A major episode of magmatism, deformation and metamorphism affected the Kenora and Daniels Lake areas between about 2710 and 2700 Ma. (4) A younger event caused deformation, metasomatism and amphibolite to granulite grade metamorphism at Cedar Lake and Daniels Lake at about 2680 Ma. (5) A subsequent, protracted period of low grade activity reset or (re-)crystallized titanite and apatite defining ages that scatter between about 2640 and 2520 Ma at Cedar and Daniels Lake but not in Kenora where titanite closed by about 2690 Ma. The 2680 Ma metamorphism may have been triggered in part by crustal thickening due to nappe thrusting but the subsequent period of lower grade activity requires the protracted addition of heat and/or fluids probably derived from magmatic and metamorphic processes continuing deep in the crust.The isotopic compositions of K-feldspars are relatively homogeneous and indicate mixing of Pb evolved in different reservoirs. The general enrichment in ²⁰⁷Pb with respect to normal terrestrial Pb reflects the protracted Archean evolution of the terrain.Now-coexisting minerals were formed and closed isotopically at different stages of the complex evolution and were selectively involved or excluded from isotopic equilibration with each other or with external systems such as hydrothermal fluids. This cautions against the indiscriminate interpretation of isotopic values obtained from whole rock systems in such complex terrains.     

On an unmetamorphosed iron-formation in the early precambrian of South-West Greenland

During Renzy Mines Ltd. exploration programme in 1972, an iron-formation was found in the Midternæs area north east of Ivigtut, South-West Greenland. It contains up to 32.9% Fe. The dominant mineral is greenalite, with minor amounts of magnetite, siderite and quartz. It was found in the northernmost part of the Ketilidian supracrustals (? 1800 m.y.), which are non-metamorphosed or very weakly metamorphosed. The iron-formation is suggested to be of submarine-exhalative origin, and precipitated in a moderately deep sea at low atmospheric O₂ pressure.

     

The Archean of the Baltic Shield: Geology, Geochronology, and Geodynamic settings

The Archean provinces and lithotectonic complexes of the Baltic (Fennoscandian) Shield are considered. The supracrustal complexes are classified by age: <3.2, 3.10–2.90, 2.90–2.82, 2.82–2.75, and 2.75–2.65 Ga. The data on Archean granitoid complexes and metamorphic events are mentioned briefly, whereas the recently found fragments of the Archean ophiolitic and eclogite-bearing associations are discussed in more detail. The Paleoarchean rocks and sporadic detrital grains of Paleoarchean zircons have been found in the Baltic Shield; however, the relatively large fragments of the continental crust likely began to form only in the Mesoarchean (3.2–3.1 Ga ago), when the first microcontinents, e.g., Vodlozero and Iisalmi, were created. The main body of the continental crust was formed 2.90–2.65 Ga ago. The available information on the Paleoarchean complexes of the Baltic Shield is thus far too scanty for judgment on their formation conditions. The geologic, petrologic, isotopic, and geochronological data on the Meso-and Neoarchean lithotectonic complexes testify to their formation in the geodynamic settings comparable with those known in Phanerozoic: subduction-related (ensialic and ensimatic), collisional, spreading-related, continental rifting, and the setting related to mantle plumes.     

Synthesis of Palaeozoic deformational events and terrane accretion in the Canadian Appalachians

Plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion, with some deformation associated with transform/transcurrent movements. Deformation associated with subduction varies between two end members: (1) where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes, a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over-riding plate; and (2) where continent-continent collision is occurring, a wide surface zone of accretionary deformation starts synchronously or with limited diachronism. Palaeozoic deformational events in the Canadian Appalachians correspond to narrow diachronous events in the Ordovician and Silurian, whereas Devonian, Carboniferous and Permian deformational events are widespread and broadly synchronous. Along the western side of the Canadian Appalachians, the Taconian deformational event starts diachronously throughout the Ordovician and corresponds to the north-north-west accretion of the Notre Dame, Ascot-Weedon, St Victor and various ophiolitic massifs (volcanic arc and peri-arc terranes) over cratonic North America. Within the eastern half of the Central Mobile Belt, the Late Cambrian-Early Ordovician Penobscotian deformational event corresponds to the ?south-easterly accretion of the Exploits subzone (various volcanic are and peri-arc terranes) over the Gander Zone (?continental rise). In the centre of the orogen, the Late Ordovician-Silurian Beothukan deformational event corresponds to the south-easterly accretion of the Notre Dame over the Exploits-Gander subzones. Along the south-eastern side of the Central Mobile Belt, the Silurian Ganderian deformational event corresponds to the north-north-east, sinistral transcurrent accretion of the Avalon Composite Terrane (microcontinent) over the Gander-Exploits zones. Along the south-eastern half of the orogen, the Late Silurian-Middle Devonian Acadian deformation event corresponds to the westerly accretion of the Meguma terrane (intradeep or continental rise) over the Avalon Composite Terrane. Affecting the entire orogen, the Late Devonian, Carboniferous and Permian, Acadian-Alleghanian deformational events correspond to the east-west convergence between Laurentia and Gondwana (continent-continent collision).     

Diverse signatures of deformation, pressure-temperature and anatexis in the Rayagada sector of the Eastern Ghats granulite terrane

The granulites and granitoids around Rayagada in the north central part of the Eastern Ghats belt display structural and petrological differences when compared to similar rocks from Chilka and Jenapore in the northern Eastern Ghats. The impress of F₁ deformation is almost erased while that ofF ₃ is muted. The metapelites have a restricted chemical range and are non-migmatitic. There are two varieties of leptynitic granitoids, one of which is interlayered with yet another S-type granite containing cordierite. The maximum recorded temperature from geothermometers is 780‡C, but the magnitude of pressure is comparatively low, the highest value being 6.3 kbar. Another distinctive feature of the pressuretemperature record is the absence of evidence of decompression in the lower realms of pressure and temperature. Metamorphic reactions that could be identified indicate cooling, a noteworthy reaction being the sillimanite to andalusite transformation. Integration of data from pressure-temperature sensors suggest cooling at two pressures, 6 and 5 kbar. The generation of two types of granitoids from metapelites is interpreted to be due to intersection with solidus curves for pelitic and graywacke-like compositions, constrained by recent experiments, at 6 and 5 kbar. The first melting occurred on a prograde path while the second one was due to increase in temperature during exhumation at tectonic rates. Thus inspite of a broad similarity in the geodynamic scenario across the northern part of the Eastern Ghat belt, differences in exhumation rates and in style of melting were responsible for producing different signatures in the Rayagada granulite terrane.     

Post-orogenic exhumation of an auriferous terrane: the paleoplacer potential of the Early Carboniferous St. Marys Basin, Canadian Appalachians

The St. Marys Basin of mainland Nova Scotia, Canada, consists of Late Devonian–Early Carboniferous clastic rocks of the Horton Group deposited in the waning stages of the ca. 420–360 Ma Acadian orogeny in the Canadian Appalachians. Clast lithologies and lithogeochemical analyses indicate that the detritus was predominantly derived from the Meguma terrane that occurs to the south of the basin. The Meguma terrane contains abundant mesothermal gold deposits that are coeval with peak magmatic activity from ca. 380 to 370 Ma and underwent rapid uplift and erosion between ca. 370 and 360 Ma. Within the St. Marys Basin, the contact between the lacustrine Little Stewiacke River Formation and the fluviatile Barrens Hills Formation is interpreted to represent a shoreline and a potentially favorable environment for depositing paleoplacer gold. Geochemical analyses of lithologies adjacent to this contact indicate that the siltstones are predominantly derived from Meguma terrane metasedimentary rocks, whereas the sandstones and conglomerates are predominantly derived from Meguma terrane granitoids. Geochemical and mineralogical analysis indicate the accumulation of heavy minerals including zircon and gold. Micron-scale (<150 μm) gold identified in the matrix of the conglomerates has a “nuggety” appearance and flakey microtexture indicative of a detrital origin. This observation indicates that the mesothermal deposits were exhumed by the latest Devonian, consistent with regional tectonic syntheses that invoke rapid uplift of the Meguma terrane following peak orogenic activity. This study suggests that favorable depositional environments for paleoplacer deposits may occur in Horton Group rocks that are derived from the Meguma terrane. Received: 27 May 1999 / Accepted: 16 May 2000     

Geochemistry and metamorphic parameters of rocks in the Batomga granite-greenstone terrane, Aldan Shield

The protoliths of the Early Proterozoic metamorphic complex in the Batomga granite-greenstone terrane are proved to comprise two petrochemical series of volcanic rocks: calc-alkaline and komatiite-tholeiite. The metavolcanic rocks of the calc-alkaline series are metamorphosed basalts, andesites, dacites, and rhyolites. The topology of the trace-element patterns of the acid volcanics is similar to that of Archean gray gneisses in platform basements, and this suggests that the petrologic mechanisms that produced the protoliths could be similar. The metavolcanics of the komatiite-tholeiite series are determined to include komatiite and tholeiite basalts. Their chemical composition is consistent with the fractionation model of high-Mg basalts in intermediate chambers under low pressures. The Nb, Y, and Zr concentrations of the metatholeiites testify that their parental melts were derived from a plume source. The metamorphic culmination parameters of the rocks corresponded to the boundary between the amphibolite and granulite facies of elevated pressure.     

胶北地块早元古代钙硅酸盐岩与高压基性麻粒岩成因及地质意义

胶北地块发育一套早元古代变质杂岩,主要由浅变质到中高级变质的滨海相至浅海相的沉积岩系列组成,属于胶-辽-吉构造活动带的一部分。其中具有顺时针变质作用PT轨迹的高压基性麻粒岩和高压泥质麻粒岩指示了造成该带闭合的碰撞构造过程。本文研究与高压基性麻粒岩密切共生的钙硅酸盐岩。首先根据矿物反应关系研究,确定钙硅酸盐岩是由含石榴石的基性麻粒岩经钙质交代形成的。进一步采用残留单斜辉石-石榴石组合、退变矿物组合,并根据SACMF体系矿物组合演化的PT视剖面图,计算得出钙硅酸盐岩经历了早期温压条件为780~850℃、1.0~1.1GPa的变质作用和晚期温压条件为400~650℃、0.6~0.75GPa的退变质作用和交代作用,从而得出顺时针的P-T轨迹。这一结果表明,钙硅酸盐岩形成于陆-陆碰撞过程的晚期阶段,是高压基性麻粒岩在折返过程中经历退变质和Ca质交代联合作用的结果。钙硅酸盐岩的形成机制及其与石榴石基性麻粒岩原岩的成因联系的确立,丰富了对于碰撞变质地块抬升过程及其岩石学效应的认识。     

基性麻粒岩造岩矿物微量元素再分配特征及其对变质历史的指示作用:以胶北地体高压麻粒岩为例

高级变质岩的变质历史是反演地壳构造-热事件的重要依据,然而高温扩散和重结晶作用能够改造造岩矿物中的主量元素分布,这对峰期变质温压条件的反演产生很不利的影响。相对于主量元素,微量元素,尤其是离子半径较大的REE,由于其在晶格中的扩散速率远小于主量元素,在高级叠加变质过程有可能记录前期变质作用。本文以胶北地体的高压基性麻粒岩为研究对象,通过详细的岩相学和矿物化学分析,初步解析了变质重结晶过程中的矿物微量元素再分配特征及其对变质作用的指示意义。岩相学上的证据表明这些样品经历了麻粒岩相变质和后期重结晶作用。单矿物的原位化学成分分析,峰期矿物石榴石、单斜辉石的主量元素Mg、Fe、Ca等二价阳离子分布均一,但部分稀土元素及微量元素则表现出钟形剖面环带分布,暗示主量元素遭受到成份扩散及重结晶所致的元素再分配,微量元素可记录峰期历史。结合主、微量元素温压计,我们分别估算了胶东基性高压麻粒岩的峰期(828℃、1.27GPa)和中压麻粒岩相退变质温压条件(810~840℃、0.6~1.0GPa),并推测其后期经历过角闪岩相退变质叠加。结合前人的年代学工作,我们认为该基性麻粒岩经历了近等温快速减压的变质历史。     

华北克拉通清原地体新太古代和古元古代两期麻粒岩相变质作用

对华北克拉通新太古代和古元古代的构造模式有多种不同认识,有必要进行深入的变质作用研究。本文选取了辽北清原地区中性麻粒岩、石榴方辉石岩和变质基性岩墙,开展系统的岩相学观察、矿物化学分析、相平衡模拟和锆石-独居定年研究,以阐明其变质演化过程和大地构造意义。中性麻粒岩和石榴方辉石岩均发育两期麻粒岩相组合。中性麻粒岩第一期斜长石发育复杂成份环带,从核到幔部其钙长石含量(X_An)降低,然后再向边部升高;石榴方辉石岩中第一期石榴石与斜方辉石互相包裹。第一期麻粒岩相变质作用P-T轨迹为逆时针型,包含峰期前升压至峰期和峰后降温降压至固相线两个变质阶段。依据两个样品中观测的峰期矿物组合在P-T视剖面图中的稳定范围,并结合斜长石幔部成份,确定峰期温压条件为1.0~1.2GPa/890~1000℃。石榴方辉石岩在峰前升压过程,斜方辉石转变为石榴石,形成石榴石包裹斜方辉石的结构;在峰后降温降压过程中,石榴石又转变为斜方辉石,导斜方辉石包裹石榴石。中性麻粒岩和石榴方辉石岩的第二期组合以形成石榴石+石英和黑云母+石英±单斜辉石±钾长石后成合晶和冠状体为特征。变质基性岩墙只发育第二期矿物组合,为高压麻粒岩组合,其P-T轨迹为顺时针型。根据变质斜长石中最小X_An,角闪石中最大Ti含量和石榴石幔部最高镁铝榴石含量等值线,确定峰期温压条件为~1.15GPa/830℃。锆石定年表明第一期高温-超高温麻粒岩相变质作用峰后冷却时间为2.49~2.48Ga,第二期高压麻粒岩相变质时间为~1.83Ga,独居石定年获得峰后退变质年龄为~1.75Ga。结合其他区域地质特征,本文认为第一期高温-超高温麻粒岩相变质作用受太古宙特有的垂直沉落构造控制,第二期高压麻粒岩相变质作用与沿华北克拉通北缘发生的碰撞造山事件有关。