Geochronology of the Phanerozoic granitoids in northeastern China

A-type granites in northeastern China: age and geochemical constraints on their petrogenesis

The Central Asian Orogenic Belt (CAOB), also known as the Altaid Tectonic Collage, is characterised by a vast distribution of Paleozoic and Mesozoic granitic intrusions. The granitoids have a wide range of compositions and roughly show a temporal evolution from calcalkaline to alkaline to peralkaline series. The emplacement times for most granitic plutons fall between 500 Ma and 100 Ma, but only a small proportion of plutons have been precisely dated. The Nd-Sr isotopic compositions of these granitoids suggest their juvenile characteristics, hence implying a massive addition of new continental crust in the Phanerozoic. In this paper we document the available isotopic data to support this conclusion. Most Phanerozoic granitoids of Central Asia are characterised by low initial Sr isotopic ratios, positive eNd(T) values and young Sm—Nd model ages (TDM) of 300-1200 Ma. This is in strong contrast with the coeval granitoids emplaced in the European Caledonides and Hercynides. The isotope data indicate their ‘juvenile’ character and suggest their derivation from source rocks or magmas separated shortly before from the upper mantle. Granitoids with negative eNd(T) values also exist, but they occur in the environs of Precambrian microcontinental blocks and their isotope compositions may reflect contamination by the older crust in the magma generation processes. The evolution of the CAOB is probably related to accretion of young arc complexes and old terranes (microcontinents). However, the emplacement of large volumes of post-tectonic granites requires another mechanism, probably through a series of processes including underplating of massive basaltic magma, intercalation of basaltic magma with lower crustal granulites, partial melting of the mixed lithologic assemblages leading to generation of granitic liquids, followed by extensive fractional crystallisation. The proportions of the juvenile or mantle component for most granitoids of Central Asia are estimated to vary from 70% to 100%.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0263593300007367

Granitoids of the Central Asian Orogenic Belt and Continental Growth in the Phanerozoic

Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications.

Abstract Asia is the world’s largest composite continent, comprising numerous old cratonic blocks and young mobile belts. During the Phanerozoic it was enlarged by successive accretion of dispersed Gondwana-derived terranes. The opening and closing of palaeo-oceans would have inevitably produced a certain amount of fresh mantle-derived juvenile crust. The Central Asian Orogenic Belt (CAOB), otherwise known as the Altaid tectonic collage, is now celebrated for its accretionary tectonics and massive juvenile crustal production in the Phanerozoic. It is composed of a variety of tectonic units, including Precambrian microcontinental blocks, ancient island arcs, ocean island, accretionary complexes, ophiolites and passive continental margins. Yet, the most outstanding feature is the vast expanse of granitic intrusions and their volcanic equivalents. Since granitoids are generated in lower-to-middle crustal conditions, they are used to probe the nature of their crustal sources, and to evaluate the relative contribution of juvenile v. recycled crust in the orogenic belts. Using the Nd-Sr isotope tracer technique, the majority of granitoids from the CAOB can be shown to contain high proportions (60 to 100%) of the mantle component in their generation. This implies an important crustal growth in continental scale during the period of 500–100 Ma. The evolution of the CAOB undoubtedly involved both lateral and vertical accretion of juvenile material. The lateral accretion implies stacking of arc complexes, accompanied by amalgamation of old microcontinental blocks. Parts of the accreted arc assemblages were later converted into granitoids via underplating of basaltic magmas. The emplacement of large volumes of post-accretionary alkaline and peralkaline granites was most likely achieved by vertical accretion through a series of processes, including underplating of basaltic magma, mixing of basaltic liquid with lower-crustal rocks, partial melting of the mixed lithologies leading to generation of granitic liquids, and followed by fractional crystallization. The recognition of vast juvenile terranes in the Canadian Cordillera, the western US, the Appalachians and the Central Asian Orogenic Belt has considerably changed our view on the growth rate of the continental crust in the Phanerozoic.

The Central Asian orogenic belt and growth of the continental crust in the Phanerozoic

A combined field, stable isotope, and whole-rock chemical study was made on late Cretaceous to Tertiary metasomatic shear zones cutting Hercynian gneisses in the Aston Massif, Pyrenees, France. Mylonitisation occurred during the early stages of Alpine compression under retrograde conditions at 400–450°C and about 10 km depth. Whole-rock δ18O values of (+11 to +12‰ in the gneisses) was lowered to +5 to +9‰ in the shear zones, with the quartz-muscovite 18O/16O fractionations of about 2 to 4‰ essentially unchanged. These 18O/16O systematics, together with δD muscovite=-40 to-50‰ indicate that large volumes of formation waters or D-rich meteoric waters passed through the shear zones during deformation. The same fluids also redistributed major elements, as shown by the correlation of δ18O shift with muscovitisation and albitisation reactions in granitic wall rocks. However, even though δ18O was universally lowered within the shear zones, the 18O/16O ratios were not homogenised, nor do they correlate in detail with the presence or absence of muscovitisation, suggesting that fluid flow was probably fracture-controlled and episodic. Field mapping shows that, along the length of a particular shear zone, muscovitisation of granite gneiss dies out 150m above the contact with underlying sillimanite gneiss. Thus, muscovitisation and albitisation of granite gneiss in shear zones and their wall rocks probably occurred during re-equilibration of acidic, chloride-rich, aqueous fluids that had previously moved upward within the shear zones through underlying sillimanite gneiss. Extremely high material-balance fluid-rock ratios (∼103) are required to explain the extent of muscovitisation along this shear zone, implying integrated fluid mass fluxes of about 108 kg/m2; this is probably close to the maximum value for other shear zones in the network. Similar volumes of a more chemically evolved fluid must have passed through the unmuscovitised mylonites, showing that the absence of alteration cannot necessarily be used to infer low values of fluid flux. For reasonable pressure gradients and time scales of fluid movement, effective permeabilities of 10-15 to 10-17 m2 are required. Such values can be accounted for by short-lived, widely-spaced cracks produced during seismic activity. A model is presented in which formation waters were seismically pumped down an underlying, shallow, southward-dipping decollement and then upward through the steeply-dipping shear zone network.

Deep fluid circulation in alpine shear zones, Pyrenees, France: field and oxygen isotope studies

Stable isotope analyses of rocks and minerals associated with the detachment fault and underlying my- lonite zone exposed at Secret Creek gorge and other localities in the Ruby-East Humboldt Range metamor- phic core complex in northeastern Nevada provide con- vincing evidence for meteoric water infiltration during mylonitization. Whole-rock 6180 values of the lower plate quartzite mylonites (>95% modal quartz) have been lowered by up to 10 per rail compared with struc- turally lower, compositionally similar, unmylonitized material. Biotite from these rocks has 6D values ranging from -125 to -175, compared to values of -55 to -70 in biotite from unmylonitized rocks. Mylonitized leucogranites have large disequilibrium oxygen isotope fractionations (Aquartz_felaspa r up to "-,8 per mil) relative to magmatic values (Aquartz_feldspa r ~ 1 to 2 per mil). Me- teoric water is the only major oxygen and hydrogen res- ervoir with an isotopic composition capable of generat- ing the observed values. Fluid inclusion water from un- strained quartz in silicified breccia has a 6D value of -119 which provides a plausible estimate of the 6D of the infiltrating fluid, and is similar to the isotopic composition of present-day and Tertiary local meteoric water. The quartzite mylonite biotites would have been in equilibrium with such a fluid at temperatures of 480- 620 ~ C, similar to independent estimates of the tempera- ture of mylonitization. The relatively high temperatures required for isotopic exchange between quartz and water, the occurrence of fluid inclusion trails and de- formed veins in quartzite mylonites, and the spatial asso- ciation of the low-180, low-D rocks with the shear zone all constrain isotopic exchange to the mylonitic (plastic) deformation event. These observations suggest that a significant amount of meteoric water infiltrated the shear zone during mylonitization to depths of at least 5 to 10 km below the surface. The depth of penetration

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Oxygen and hydrogen isotope evidence for meteoric water infiltration during mylonitization and uplift in the Ruby Mountains-East Humboldt Range core complex, Nevada

A Stable Isotope Study of Anorogenic Magmatism in East Central Asia

During the Phanerozoic, granitoid magmatism occurred on a vast scale in Transbaikalia and Mongolia in east central Asia. Within this huge and geologically complex region, many hundreds of individual plutons of syenite, quartz syenite, quartz monzonite, and K-rich granite have been intruded within NE-SW oriented belts 200–400 km wide and 2000–3000 km long. Five compositionally distinct, major stages of magmatism are distinguished: (1) Ordovician-Silurian, (2) Devonian, (3) Early Permian I, (4) Early Permian II, and (5) Permo-Triassic. Granites and syenites occur within each suite, but the proportion of syenite increases with time, as does the alkalinity of all magmas. Synplutonic mafic rocks are associated with plutons of all five suites and mafic/felsic magma-mixing textures are common. Laser fluorination oxygen isotope analysis (Sharp, 1990) of titanite from granitoids indicates that within and among plutons of a given suite 18O/16O is fairly constant. This indicates that these magmas probably formed with a high degree of isotopic (and chemical) homogeneity. However, there is a progressive decrease in δ18O of titanite with time from about +6.5 in the earliest suite to about +1.5 in the youngest suite (corresponding to a decrease in magmatic δ18O from ∼+11 to ∼+6). The systematic evolution of geochemical and isotopic composition with time allows us to develop a model explaining the progressive increase in alkalinity (especially K2O) and decrease in δ18O over 250 m.y. of plutonism. These constraints may be satisfied by progressive hybridization of K-rich (alkali) basalt magmas with crustally derived silicic melts, accompanied by fractional crystallization of K-poor phases such as plagioclase and pyroxene (cf. Barker et al., 1975). Such a mechanism could form large volumes of syenitic residual liquids, having the appropriate isotopic composition. Younger suites were in part derived through remelting and hybridization of material formed or intruded during earlier magmatic episodes, leading to a progressive increase in overall alkalinity and decrease in δ18O. The repeated cycles of magmatism involved significant anorogenic crustal growth and a gradual cratonization of the continental crust of East Asia, caused by multiple melting and remelting events and associated large influxes of mantle-derived alkalic mafic magma.

Geochemical evolution of Phanerozoic magmatism in Transbaikalia, East Asia: A key constraint on the origin of K‐rich silicic magmas and the process of cratonization

Regional carbonate alteration of the crust associated with major shear zones provides direct evidence for $CO_{2}$-rich fluid mobility. A good example occurs within the Attur lineament, one of numerous Proterozoic shear zones which crosscut charnockitic gneisses and other lithologies of the southern Indian craton. At this locality, widespread carbonate alteration of silicate rocks has involved growth of ankerite and other carbonate minerals which replace preexisting silicates. Some of the ankerite was subsequently recrystallized to fine-grained calcite and iron oxide, accompanied by sericitization of feldspar. Carbon, oxygen, and strontium isotope analyses of samples of ankerite and calcite, together with some of the coexisting silicate minerals, have been used to constrain the conditions of formation of the carbonate alteration and the origin of the fluids involved. $\delta^{13}C$ values of ankerite are relatively homogeneous, ranging between -6.5 and -3.9 ‰ with a mean of -5.3 ‰. $\delta^{18}O$ is also ...

Stephen M. Wickham

Papers

Deep fluid circulation in alpine shear zones, Pyrenees, France: field and oxygen isotope studies

Oxygen and hydrogen isotope evidence for meteoric water infiltration during mylonitization and uplift in the Ruby Mountains-East Humboldt Range core complex, Nevada

A Stable Isotope Study of Anorogenic Magmatism in East Central Asia

Geochemical evolution of Phanerozoic magmatism in Transbaikalia, East Asia: A key constraint on the origin of K‐rich silicic magmas and the process of cratonization

Regional Carbonate Alteration of the Crust by Mantle-Derived Magmatic Fluids, Tamil Nadu, South India