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Accretionary Orogens and Growth of the Continental Crust from Neoproteozoic to Recent: Nd-Sr Isotopic Evidence
Bor-ming Jahn ( 江博明 )
Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan 11529; Department of
Geological Sciences, National Taiwan University, Taipei (jahn@earth.sinica.edu.tw)
Accretionary orogens constitute major sites of continental growth and mineralization. They include Archaean greenstone belts, Proterozoic orogens (e.g., the
Birimian of West Africa, Svecofennian of S. Finland, Cadomian of NW Europe, and the
Arabian-Nubian shield), Neoproterozoic-Mesozoic orogens in Central Asia (CAOB), as well as Paleozoic to Recent orogens of the circum-Pacific and Caribbean. Accretionary orogens form at sites of subduction of oceanic lithosphere. They consist of accretionary wedges containing material accreted from the downgoing plate and eroded from the upper plate, plus island arcs, ophiolites, oceanic plateaus, old continental blocks, metamorphic rocks and syn- and post-orogenic granitoids. To evaluate the mechanism of crustal generation and to estimate the proportion of juvenile crust in any given orogen, we commonly study granitic rocks using the radiogenic (Sr-Nd-Pb) and stable (O) isotope tracer techniques in addition to field and petrological investigations. The following discussion will be based mainly on the available Nd-Sr isotopic studies.
(1) The Central Asian Orogenic Belt (CAOB) or the Altaid Tectonic Collage (Sengor et al.,
1993). It is a prime example of accretionary orogeny and represents a complex evolution of Neoproterozoic to Mesozoic orogenic belts. A common characteristic feature of the individual orogens in Central Asia is the complex but recurrent arrangement of accretionary prism and magmatic arcs, interspersed with massifs of Precambrian terranes and slivers of oceanic crust. Early Paleozoic UHP and HP metamorphism occurred in
Kazakhstan (Kokchetav) and the Gorny Altai, respectively. In addition, all the orogens are intruded by massive granitoids of early Paleozoic to late Mesozoic ages. Overall, the
CAOB grew southward from the Siberian Craton. The final closure of the Paleo-Asian
Ocean probably took place in the late Permian or Permo-Triassic time when the North
China Craton collided with the CAOB. Nd isotope tracer studies in the last decade revealed that massive juvenile crust was generated in the CAOB. While this being true, continued isotope analyses also showed that in some terranes Precambrian crust has also played an important role in the generation of the apparent "juvenile" crustal mass. This is illustrated in Mongolia, Altai-Xinjiang, Inner Mongolia, NE China, Transbaikalia,
Kazakhstan, and West Sayan and Gorny Altai of southern Siberia (e.g., Jahn, 2004;
Kovalenko et al., 2004). The juvenile crust was likely produced in two ways: by lateral accretion of arc assemblages and syntectonic granitoids, and vertical accretion of nonorogenic granitoids and their volcanic equivalents. The vertical accretion was probably achieved by underplating of mantle-derived basic rocks, followed by partial melting of these rocks and later intracrustal differentiation. However, ocean closure and arc accretion are still considered to be the dominant process of crustal formation in the CAOB.
Available Nd isotope data indicate that the proportion of the mantle component in postorogenic granitoids appears to be greater than that in syntectonic granitoids.

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(2) The Arabian-Nubian Shield (ANS) has much in common with the CAOB regarding their tectonic and crustal evolution. ANS is a large Neoproterozoic accretionary orogen at the northern end of the East African Orogen. ANS underwent the main stage of crustal accretion in the period of ca. 900 to 530 Ma (= Pan-African event), whereas CAOB took place later, from ca. 600 to ca. 100 Ma. Both CAOB and ANS are composed of a variety of terranes, including continental-margin and intra-oceanic arcs, ophiolite complexes, granitic batholiths and some Precambrian continental fragments. However, the most renowned feature is the massive generation of juvenile crust in the respective periods of
Neoproterozoic for ANS and Phanerozoic for CAOB. In southern Israel (northern part of
ANS), late Pan-African dike swarm of basic to acid compositions were emplaced in several episodes from 600 to 530 Ma. Late Pan-African dike swarms are abundant throughout the Sinai Peninsula and the Eastern Desert of Egypt, and their geochemical characteristics are comparable with the dike rocks occurring in southern Israel. All these dike rocks were formed in post-orogenic and within-plate tectonic settings. They are enriched in alkalis, LILE and HFSE; and in some basalts, high TiO
2 contents (4.5%) are observed. New Sr-Nd-O isotope analyses on the dike rocks from southern Israel indicate a significant proportion of juvenile component in their petrogenesis. However, a very wide range of radiogenic I(Sr) values of 0.704 to 0.760 and oxygen isotope analyses suggest that severe post-magmatic water-rock interaction has taken place.
The Katharina Ring Complex is one of the most representative Late Pan-African ring complexes in the Arabian-Nubian Shield. Along with numerous A-type granite plutons, it was formed in the last stage of the Shield evolution at about 600 Ma. The
Complex is situated in the high mountainous area of the Sinai Peninsula (Gebel Musa), which is also known as the biblical Moses Rock in Mount Sinai. Like in many cases in
CAOB, the Katharina Ring Complex has very low initial 87 Sr/ 86 Sr values (0.702–0.7035) and positive ε
Nd
(T) values, pointing to their ‘juvenile’ character and to that granitic magmas were derived by partial melting of juvenile rocks or by differentiation of mantlederived magma. Granitic rocks and rhyolites from other parts of the Arabian-Nubian
Shield consistently show positive ε
Nd
(T) values, which provide indisputable evidence for the juvenile nature of the Arabian-Nubian Shield, and has been often used to advance the notion of high rate of crustal growth in the Neoproterozoic (e.g., Stern and Kröner, 1993;
Stern, 2002). The similar T
DM values of about 900-1000 Ma for the alkaline granites and for the crust in the Arabian-Nubian Shield are in full agreement with the juvenile character of the Katharina pluton.
(3) The Japanese Islands represent a segment of a 450 Ma old subduction-related orogen developed along the western Pacific convergent margin, and most tectonic units are composed of late Paleozoic to Cenozoic accretionary complexes and their high P/T metamorphic equivalents (Isozaki, 1996). The formation of the Japanese Islands has been taken as a standard model for the accretionary orogeny and often serves as an example for understanding the crustal evolution of the Central Asian Orogenic Belt (CAOB).
According to Maruyama (1997), the most important cause of the orogeny is the subduction of oceanic ridge, by which the continental mass increases through the transfer of granitic melt from the subducting oceanic crust to an orogenic belt. Sengor and Natal’in (1996) named the orogenic complex "Nipponides", consisted predominantly of Permian to Recent subduction-accretion complexes with very few fragments of older continental crust.
These authors pointed out the resemblance in orogenic style between Japan and CAOB, and further emphasized the large proportion of the volume of accretionary complex with

3 respect to the volume of older crust. It is, by implication, that the accretionary complex in
Japan must be quite juvenile as a whole. In this scenario, voluminous granitic melts emplaced in SW Japan and elsewhere in the Japanese Island would show Sr-Nd isotopic signatures indicative of juvenile crust as observed in many parts of Central Asia or the
Arabian-Nubian Shield. In the first place, this appears to be consistent with the absence of
Precambrian rocks in Japan. However, the available Nd-Sr isotopic data, compiled from the literature and our new analyses on Cretaceous granites from the Sanyo Belt, Miocent granites from the Shimanto Belt, and Quaternary granites from the Japan Alps, indicate that they are in strong contrast with those observed in CAOB and ANS. In fact, the isotopic signatures are more comparable with those observed in SE China and Taiwan, or in classical collisional orogens in the European Hercynides and Caledonides. A large proportion of Mesozoic and Cenozoic granitoids from SW Japan have Proterozoic Sm-Nd model ages, high initial
87
Sr/
86
Sr ratios and negative ε
Nd
(T) values. This raises questions about the types of material accreted in accretionary complexes, and the validity of the hypothesis that the Nipponides contains very few fragments of older continental crust. We conclude that the subduction-accretion complexes in Japan are composed mainly of recycled continental crust, probably of Proterozoic ages. The scenario is comparable with that in Taiwan. The proportion of mantle-derived juvenile crust in the edifice of the
Japanese Islands is much smaller than that in the CAOB.
Conclusion. Accretionary orogens are generally characterized by abundant distribution of juvenile crust, and represent the major sites of continental growth (and mineralization, not discussed) in the history of the Earth. However, the isotopic evidence also shows that preexisting Precambrian crust has played different roles in the generation of crustal mass in individual orogens or terranes (Jahn et al., 2004; Kroener et al., 2007; Wang et al., 2007).
The proportion of the juvenile or mantle component in accretionary orogens is highest in the Arabian-Nubian Shield, followed by the Central Asian Orogenic Belt and lowest in the
Japanese Islands (supported by NSC grants 93-2116-M-001-025, 94-2116-M-001-021, 95-
2116-M-001-021).