Ordovician and Carboniferous Volcanism/Plutonism in Central Inner Mongolia, China and Paleozoic Evolution of the Central Asian Orogenic Belt

Adakite was originally proposed as a genetic term to define intermediate to high silica, high Sr/Y and La/Yb volcanic and plutonic rocks derived from melting of young, subducted lithosphere. However, most volcanic rocks in modern island arcs and continental arcs are probably derived from melting in the mantle wedge. Trace element chemistry with high Sr/Y ratios is a distinguishing characteristic of adakites. Ordovi‐ cian and Carboniferous volcanic/plutonic rocks with high Sr/Y ratios occur in Central Inner Mongolia, which is situated on the southern margin of the Central Asian Orogenic Belt (CAOB). The samples are mostly granodiorite, tonalite and quartz-diorite in composition with intermediate to high-silica, high Na 2 O (3.08–4.26 wt.%), low K 2 O (0.89–2.86 wt.%) and high Na 2 O/K 2 O and Sr/Y ratios. Their chondrite-normalized REE patterns are characterized by LREE enrichment. In mantle-normalized multi-element variation diagrams, they show typical negative Nb anomalies, and all samples display positive ε Hf (t) and ε Nd ( t ) values, and low I Sr . The Ordovician rocks, however, show higher Sr/Y and La/Yb ratios than the Carboniferous samples, implying that the older granitoids represent adakitic granitoids, and the Carboniferous granitoids are typical subduction-related arc granitoids but also with adakite-like compositions. The results are compatible with the view that the Central Asian Orogenic Belt (CAOB) in Inner Mongolia evolved through operation of several subduction systems with different polarities: an early-middle Paleozoic subduction and accretion system along the northern margin of the North China Craton and the southern margin of the Mongo‐ lian terrane, and late Paleozoic northward subduction along the northern orogen and exhumation of a high-pressure metamorphic terrane on the northern margin of the North China Craton.

Adakite was originally proposed as a genetic term to define intermediate to high-silica, high Sr/Y and La/Yb volcanic and plutonic rocks derived from melting of young, subducted lithosphere [16]. However, most volcanic rocks in modern island arcs and continental arcs are probably derived from melting in the mantle wedge [17]. Trace element chemistry with high Sr/Y ratios is a distinguishing characteristic of adakites [16,18]. Ordovician and Carboniferous volcanic/plutonic rocks with high Sr/Y ratios occur in Central Inner Mongolia, which is situated on the southern margin of the Central Asian Orogenic Belt (CAOB, [19]). Early Paleozoic [6][7][8][9][20][21][22] and Late Paleozoic [2][3][4]23] arc volcanism/plutonism as part of trench-island arcbasin systems occurred along the southern margin of South Mongolian microcontinent and the northern margin of North China Craton, suggesting concurrent two-way subduction towards opposing continental margins. The chapter focuses on early and late Paleozoic volcanic/plutonic rocks with high Sr/Y ratios in Central Inner Mongolia, and contributes geochemical data to the evolution of the CAOB.
In Central Inner Mongolia and adjacent southern Mongolia, the Solonker suture zone can be traced for ca. 1000 km by dismembered ophiolite fragments (Figure 1) and represents a major paleo-plate boundary in Central Asia that stretches northeastwards for more than 2500 km in Mongolia and China [28]. It has been variably interpreted as the southernmost limit of the Altaids ( [10]) or the southernmost termination of the CAOB [1]. The Solonker suture zone separates two continental blocks (Figure 1) [3]. The Northern Block consists of the Southern Mongolia (or Hutag Uul) block (gneissic granite, 1784 ± 7 Ma, Shi et al., unpublished data) and the Northern Orogen, which includes metamorphic complex (an orthogneiss has a zircon age of 437 ± 3 Ma, [29]), an ophiolitic mélange with blueschist, a near-trench granitoid (ca. 498-461 Ma) and a juvenile arc (ca. 484-469 Ma, [3]). The Southern Block comprises the southern orogen and the northern margin of the North China Craton.  Figure 1A compiled after [19]; Figure 1B after [3,25]). In Figure 1B, the Solonker suture zone represents the tectonic boundary between the northern and the southern continental blocks [3]. Paleozoic volcanic rocks and granitoids are widely distributed along the margin of the Solonker suture zone. Ordovician granitoids (quartz-diorite, granodiorite, diorite, tonalite, and trondjemite; Table 1 and Figure 2) occur in the northern and southern orogen [7,8,20,21,42,43]; Figure 1), whereas Carboniferous volcanic rocks and granitoids (quartz-diorite, granodiorite, tonalite, and granite; Table 1 and Figure 2) are mainly distributed in the northern orogen ( [2,23,30,31,35,37,38,40,41]; Figure 1), and scattered along the northern margin of the North China Craton [44,45]. The geochemical data of representative rocks are listed in Table 2 Table 1). A for rocks from the Northern Block, which consists of the Southern Mongolia (or Hutag Uul) block and the northern orogen; and B for rocks from the Southern Block, which is composed of the northern margin of North China Craton and the southern orogen [3].  Figure 3 shows the photographs of field occurrences and photomicrographs of some representative samples. Figure 3A was taken from Central Inner Mongolia to show the beautiful landscape; Figure 3B shows the Carboniferous volcanic rocks which are located in the Southern Block.

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The Inner Mongolian Ordovician granitoids of this study have depleted HREE, Nb, positive Sr anomalies, low Y and Yb contents and positive to weakly negative Eu anomalies. These characteristics are consistent with the loss of plagioclase and the presence of garnet as residual phases, probably related to partial melting of the source material under eclogite-facies conditions [61,62]. The petrology and geochemistry of the Ordovician adakitic granitoids indicate a contribution from melting of subducted oceanic crust in their formation rather than melting of thickened basaltic lower continental crust.  Figure 4), and with low I Sr (0.7043-0.7060), positive ε Nd (t) (+1.0 to +5.1) and ε Hf (t) (+8.1 to +12.3) isotope ratios (Table 1; Figures 7 and 8). However, most of them have lower Sr and Sr/Y ratio than those of Ordovician adakitic granitoids in this area (Table 2; Figure 6), which are typical subduction-related arc granitoids [52,63,64] although still with adakite-like compositions [16,48].  Hf(t) for zircons from (data from [2], and [32] for Carboniferous granitoids).
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Geodynamic significance of the Ordovician and Carboniferous volcanic rocks and granitoids
A subduction-accretion complex usually forms along a convergent plate boundary where an oceanic plate subducts beneath another oceanic or continental plate [65]. Early Paleozoic arc plutonism as part of trench-island arc-basin systems ( [6,8,21,22]; Table 2) occurred in the southern orogen, along the northern margin of the North China Craton, and late Silurian molasse deposits unconformably overlie these rocks [6,66]. Coeval adakitic plutonism is emplaced in the northern orogen, along the southern margin of the Mongolian terrane [20]. Silurian high-pressure metamorphic rocks [67] and Silurian syncollisional magmatism in the northern orogen along the Solonker suture [68] were also reported. All these features indicate an early-middle Paleozoic subduction and accretion system along the northern margin of the North China Craton and the southern margin of the Mongolian terrane. After demise of the ocean in the southern orogen, caused by subduction of a ridge crest and by ridge collision with supra-subduction zone ophiolite in the Silurian [8], the southern orogen became tectonically consolidated and turned into a post-orogenic setting [69].
Additionally, a subduction-accretion complex was identified from previously defined late Carboniferous and early Permian strata in the Daqing pasture, southern Xiwuqi, Inner Mongolia [4]. In addition to this subduction-accretion complex, most magmatic rocks are considered to have formed in a subduction setting [23,30], and the spatial configuration of both geological units indicates that the subduction polarity was from south to north [4] along the northern orogen.
Carboniferous granitoids on the northern margin of North China craton also have the composition of tholeiitic and calk-alkaline island-arc rocks and adakitic compositions [45], however, low negative whole-rock ε Nd (t) and zircon ε Hf (t) isotope ratios indicate that they were derived mainly from anatectic melting of the ancient lower crust with some involvement of mantle materials [70]. The Carboniferous plutons were interpreted as subduction-related and emplaced in an Andean-style continental-margin arc [70].
On the northern margin of the North China craton, however, Carboniferous eclogites are exposed at least 200 km south of the Solonker suture zone and have tholeiitic protoliths (MORB and IAT), and eclogite-facies metamorphism reflects deep subduction of oceanic lithosphere [71]. The granitoids (330-298 Ma) of this area were emplaced and deformed during, and/or shortly after eclogite-facies metamorphism (ca. 331-319 Ma) [71]. This close temporal relationship indicates that magmatism closely followed the exhumation of the high-pressure metamorphic terrane [3].

A possible model for the discrete evolution of CAOB
The southeastern CAOB was formed by the concurrent two-way subduction of Paleo-Central Asian Ocean towards opposing continental margins in the early Paleozoic ( Figure 9A). In the south is an arc-trench complex, which can be regarded as an analogue of the Izu-Bonin-Mariana arc [72], and in the north a product of ridge-trench interaction [8]. In the late Paleozoic, however, Andean-type orogenesis was induced by subduction of Central Asian Ocean beneath either the northern (e.g. [4]) or southern (e.g. [45]) continental blocks ( Figure 9B). Plutonic magmatism [45] was accompanied by exhumation of a high-pressure metamorphic terrane [71] in the south; and a subduction-accretion complex [4], together with most arc-related magmatic rocks [23,30] was formed along the northern orogen.

Cook Island and Cerro Pampa adakites
Cenozoic andesitic to dacitic rocks collected from Cerro Pampa [51] and andesites from Cook Island [50] have intermediate to high-silica, high Al 2 O 3 , higher Na 2 O than K 2 O, low HREE, depleted HFSE, Y and Yb, high Sr, and high Sr/Y ratios (Table 3), and low I Sr with positive ε Nd isotope ratios. The samples, therefore, represent adakites [50,51]. Cerro Pampa adakitic magmas formed in response to melting of hot slab that was subducting beneath South America [51], and similar petrogenesis for the Austral Volcanic Zone adakites [50]. Ordovician adakitic rocks from Central Inner Mongolia show similar petrogenesis and geotectonic setting with the Cenozoic adakites from Cook Island [50], Cerro Pampa [51] and Aleutian arc [16].

Oman and Chile volcanic arc granites
Volcanic arc granites from Oman and Chile have high-silica, intermediate Al 2 O 3 , low HREE [52] (Table 3), and with low Sr and Sr/Y ratios than the adakites ( Table 3), which are typical subduction-related arc granitoids derived from melting in the mantle wedge. Most Carboniferous volcanic rocks and granitoids present similar petrogenesis and geotectonic setting with the Cenozoic subduction-related arc granitoids. 2. The Ordovician rocks show higher Sr/Y ratio than the Carboniferous rocks, suggesting that the former represent adakitic rocks and the latter are typical subduction-related arc rocks with adakite-like compositions.

3.
The Central Asian Orogenic Belt evolved through several subduction systems with different polarities in Central Inner Mongolia, namely an early-middle Paleozoic subduction and accretion system along the northern margin of the North China Craton and the southern margin of the Mongolian terrane, and late Paleozoic northward subduction along the northern orogen and exhumation of a high-pressure metamorphic terrane on the northern margin of the North China Craton.