Geochemical Characteristics and Tectonic Significance of the Acidic Volcanic Rocks from the Shetang-Boyang Area , Western Qinling Orogenic Belt , China

Acidic volcanic rocks of Shetang-Boyang area are located in the western Qinling orogenic belt, consist of rhyolite and granite porphyry. They are comparable in the chemical composition, enriched in Si, alkali, Al and a little bit of Mg, Ca and Ti. The contents of HFSE (Zr, Hf) and LILE (Rb, Th, U) are high, however, the content of Ba, Sr, Ti, P have obviously depleted and there are obvious negative Eu anomalies (Eu/Eu*=0.06-0.13). These geochemical characteristics are revealed that these volcanic rocks have an A1 type affinity. Geochemical data combined with regional studies, show that these volcanic rocks were formed in a continental extension setting and the western Qinling orogenic belt in 211Ma has been in the tectonic setting of post-collisional extension.


Introduction
Shetang-Boyang area in Tianshui city is located in the convergent juncture of the North China block, Qinling orogen, and Yangtz block, and is an important region during the western Qinling orogenic belt tectonic evolution.Previous studies have shown that there are terrestrial acidic volcanic rocks developing in Shetang-Boyang area (Pei et al., 2004;Ding et al., 2005;Lai et al.,2006;Xu et al.,2007), and achieved certain results, however, the geochemistry and tectonic genesis remain controversial.Some scholars believed that the volcanic rocks belong to nonorogenic alkaline rocks, comparable to continental rift type alkaline rhyolite, were formed by partial melting of crustal rocks in continental extension setting, related to sinistral strike-slip shearing of Weihe faulted zone in Early Cenozoic (Pei et al., 2004;Ding et al., 2005.).Some workers suggested that the volcanic rocks were resulted from the Late Triassic under the tectonic setting of post-collisional of the western Qinling orogenic belt (Xu et al., 2007).Much of the debate significantly constrains on better exploring the tectonic evolution of western Qinling orogenic belt.Therefore, based on detailed field investigation, petrography and geochemistry study, this paper has done systematic research of the volcanic rocks and has finally drawn conclusions of the type of the volcanic rock, petrogenesis and tectonic significances.This provides important fundamental data for the tectonic evolution of western Qinling orogenic belt, China.

Geological Settings and Samples
The study area is located in the north margin of western Qinling orogenic belt, related to the mid segment of the Central orogenic belt, also is the convergent juncture of the North China block, Qinling orogen, and Yangtz block, and is an important region during the western Qinling orogenic belt tectonic evolution (Feng et al., 2003;Sun et al., 2004;Pei et al., 2007 and2009;Liu et al., 2014;Zhang et al., 2005).Affected by the long-term activity of the Tianshui-Baoji fault, the area has multiple zones of tectonic activity.Since the Meso-Cenozoic, because of the collision between the Indian plate and the Eurasian plate, and the northeast extension of the Qinling-Tibet Plateau, the structures are strongly deformed and strike-slip has widely developed (Zhang et al.,2001;Pei et al., 2004).
The acidic volcanic rocks are composed of lava, tuff, hypabyssal granite porphyry and rhyolite, dominantly occurred at the Shetang-Boyang-Yuanlong-Putaoyuan region with the acreage of 34km 2 , extend along a north-south orientation (Fig. 1).The volcanic rocks extend southward to meet the Paleoproterozoic Qinling Group, northward to join Paleozoic Huluhe group, and fault contact with The Indo-Chinese epoch monzogranite.
The quartze sysenite-porphyry invaded into acidic volcanic rocks and Qinling group.
To ensure the accuracy of the results, four representative samples, including two rhyolites (No.TSP1 and TSP4) and two hypabyssal granite porphyry (No.TSP7 and TSP8) were chosen for studying, and excluded the contaminated samples.

Analytical Methods
Rocks were grinded into thin sections, and study under the Polarizing microscope, research there optical properties, structure, and ensure the features, genesis of rocks.
Geochemical analyses were undertaken at the Laboratory of Mineralization and Dynamics, Chang'an University (Xi'an, China).The major element compositions were analyzed by X-ray fluorescence spectroscopy (XRF-1500).Trace elements and rare earth elements (REE) were measured by ICP-MS.

Petrography
Microscopic thin section analysis indicates that the phenocrysts mainly consist of plagioclase, orthoclase and quartz, and the content proportion of the three is nearly the same.The matrixes are quartz-feldspathic, with rhyolitic structure and felsitic texture, and the accessory mineral is corundum.

Geochemical Analyses
Major and trace element compositions of the analyzed samples are listed in Table 1.

Major Elements
According to the observation under the microscope and geochemical characteristics, the samples can be divided into two types of granite: rhyolite and granite porphyry.

Trace elements
Rhyolite and granite porphyry originate from single magma, and they have similar characteristics of trace elements, so we analyze them together.
The volcanic rocks are enriched in Rb (91.27-233.5 ppm), U (3.17-5.50ppm), Th (14.25-22.19ppm), and high field strength elements (HFSE) (Zr: 134.95-194.74ppm; Hf: 3.87-5.59ppm) and are depleted in Ba (64.17-339.03ppm) and Sr (34.89-71.91ppm), which are similar to A-type granite (Collins et al.,1982;Whalen et al., 1987).In A-type granite discrimination diagrams (Fig. 3), all samples of the volcanic rocks fall in the field of A-type granite.One sample only of rhyolite sample plot in I,S,M, type granite field in the relationship between 1000Ga/La with Zr (ppm), which resulting of low relatively content of Zr (about 135 ppm).The volcanic rocks are also enriched in Rare Earth Elements (REE) with total REE concentration of 92.08-162.33 ppm.Chondrite-normalized REE patterns show slight enrichment of Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Element (HREE) with (La/Yb) N ratios of 13.58-18.83and strongly negative Eu anomalies (Eu/Eu*=0.06-0.13)(Fig. 4a).In the element spider diagram (Fig. 4b), the volcanic rocks show negative anomalies of Ba, Sr, P, Ti and Eu, which agrees with element compositional patterns of A-type granites (Wu et al., 2002).The above geochemical characteristic, show clearly that the volcanic rocks are A-type granite.

Tectonic Setting
Eby (1992) subdivided A-type granites into A 1 and A 2 subgroup.These two types have different sources and tectonic setting.The A 1 group occurs in an orogenic setting such as continental rifts or intraplate environments, and their magmas are derived from sources like those of oceanic-island basalts.The A 2 group forms in a broader range of environments, and those emplaced at the end of a long period of apparently high heat flow and granitic magamatism.The A 2 group magmas were derived from continental crust or underplated crust that has experienced a cycle of continent-continent collision or island-arc magmatism (Zhang et al., 2007).The A 1 and A 2 groups can be discriminated using the Y-Nb-Ce andY-Nb-3*Ga diagrams (Fig. 5(a)).As shown in Figure 5(a), all samples fall into the A 1 group, suggesting a continental rifts or intraplate environments.

Geodynamic Implications
In this paper, the rhyolite and granite porphyry have a similar chemical composition (see above) and formed in a continental rifts or intraplate environments.In the Rb vs Yb+Ta tectonic discrimination diagram of Pearce et al. (1984) volcanic rocks fall into the field of "within plate granite" (Fig. 5(b)), indicate that the rocks were formed in a continental extension setting.
Combined with previous studies, the Qinling orogenic belt, being part of the central orogenic belt in China, was sutured to the Yangtz block during the middle to late Triassic (Li et al., 2009 and2010), and the end period of Indo-China Movement in Qinling is characterized by changing from continental crust thickened to lithosphere mantle adjusting or crust thining (Zhang et al., 2001;Zhang et al., 1995;Zhang et al., 1996Zhang et al., , 1997)), Gong et al. (2009) proposed that the tectonic event of transition from lithospheric extrusion to extension in Qinling orogenic belt, may generated in range of 224-210Ma.Moreover, Xu et al. (2007) zircon U-Pb dating yields ages of 211Ma for magma crystallization, corresponding to the end period of Indo-China Movement, Pb isotopic data suggest an impossible source of the North Qinling basement rocks for rhyolites.Therefore, the western Qinling orogenic belt in 211Ma has been in the tectonic setting of post-collisional extension.Note.Syn-COLG-syn-collision granite; VAG-volcanic arc granite; WPG-within plate granite; ORG-ocean ridge granite.

Conclusions
The rocks are A 1 -type granite, including one rhyolite and three granite porphyry, and formed in a continental extension setting, combined with previous studies, we consider that the western Qinling orogenic belt in 211Ma has been in the tectonic setting of post-collisional extension.

Figure 1 .
Figure1.Geological and location maps of the study area (afterPei et al., 2004)

Figure 2 .
Figure 2. (a) Plots of SiO 2 versus AR of the studied samples(after Wright J. B., 1969); (b) A/NK versus A/CNK diagram (after Maniar P D et al., 1989) (The following symbols are the same)

Table 1 .
Major (%) and trace element (ppm) of the studied volcanic samples