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

http://argonlab.gl.ntu.edu.tw/papers/Wu%20et%20al.(2003)Lithos.pdf

Highly fractionated I-type granites in NE China (I): geochronology and petrogenesis

Every geologist is acquainted with the principle of “uniformitarianism,” which holds that present-day processes are the key to those that operated in the past. But the extent this applies to the processes driving the growth and differentiation of the Earth's continental crust remains a matter of debate. Unlike its dense oceanic counterpart, which is recycled back into the mantle by subduction within 200 Ma, the continental crust comprises buoyant quartzofeldspathic materials and is difficult to destroy by subduction. The continental crust is, therefore, the principal record of how conditions on the Earth have changed, and how processes of crust generation have evolved through geological time. It preserves evidence of secular variation in crustal compositions, and thus the way in which the crust has formed throughout Earth's history. Exploring the nature and origin of these variations is the focus of this chapter.

Clément Gariépy

Papers

Geochemical and Isotopic (O, Nd, Pb and Sr) Constraints on A-type Granite Petrogenesis Based on the Topsails Igneous Suite, Newfoundland Appalachians