Statistics and Formation Mechanism Analysis on the Basin-Range Patterns of Granite Bodies in Hunan Province, China

This paper presents the results of statistical analysis on the basin-range relations between the granite rock bodies and surrounding rocks in Hunan Province of China. Among the 44 granitoids in Hunan Province, 28 are basins, 14 are ranges, and only 2 cannot be directly classied. The basin-range properties of granite bodies are closely related to the lithology of surrounding rocks. Among the 28 granite basins, 24 are surrounded by slate, 3 by sandstone, and only 1 by glutenite, while 11 of 14 ranges are surrounded by carbonate rocks. From the perspective of endodynamic process, tectonic movements played an important role in the evolution of the terrain in the granite areas. Firstly, tectonism shaped the large-scale tectonic framework which deter-mined the distribution of some granite mountains in Hunan Province. In addition, tectonic compression or extension formed some granite compressional uplifts or horsts, which present as ranges now. From the perspective of differential weathering, the difference of resistance to weathering between granite and their surrounding rocks is an essential factor for the development of granite basin or range. When their surrounding rocks are carbonate rocks, the granite areas mostly present as ranges for the high solubility of carbonate minerals. When their surrounding rocks are slate rocks, the granite areas are mostly basins for their lower resistance to weathering than slate rocks determined by their more unstable minerals to weathering and more conductive soils to rainfall inltration.


Introduction
Basin and range are the basic geomorphic forms of the earth, which are widely distributed on the land and essential parts of the gorgeous submarine topography. Basin and range evolution mechanisms have always been the forefront research topic of geology, geography, geophysics, and other geosciences that involve a wide range of elds such as tectonics (Li et al. 2013), magmatism (Wang et al. 2003), geomagnetic evolution (Sun et al. 2019), and rock weathering (Cui et al. 2019). Basin-range pattern has an obvious scale effect. The formation mechanisms of basin-range patterns within different spatial scales are often quite different. Large-scale basin-mountain systems are usually controlled by global-scale tectonic movements, among which Tibetan-Ganges Plateau-Basin system is a typical example (Powell et al. 1973;Zhou et al. 2005). On the medium scale, basins are mostly the results of local tectonic subsidence. For example, the Dongting Lake Basin, which is located in south-central China, was formed after multistage intraplate tectonic movements (Pi et al. 2001). From a small-scale point of view, the formation of basins and ranges is related to both tectonic movements and the differences of the adjacent rocks' resistance to weathering (Cui et al. 2007).
China is one of the countries with the most extensive distribution of granite masses in the world, with a total area of 909,000 km 2 , of which about 60% are distributed in southern China. Granites with different ages, genesis, and lithology types different geomorphic forms in different climatic zones, altitudes, and different areas of exogenous force.
Based on morphological laws, some scholars have systematically summarized the main geomorphic features and types of granite (Twidale 1982; Twidale et al. 2005). They have divided the main granite landforms into boulders, inselbergs, slopes, topology, and plains, and considered boulders as the primary geomorphic manifestation of granite. In China, as early as the 1960s, Zeng (Zeng. 1960) divided the granite landforms into ve types according to China's granite characteristics, which are alpine granite terrain, tropical granite terrain, dry area granite terrain, egg terrain, and gully terrain respectively. Later, Cui et al. (2007) divided the granite landform into four categories from the perspective of weathering and erosion in the formation process of granite landforms. Geomorphic system results from the interaction of internal and external forces on the surface , and the landform in granite area is also affected by three main factors, namely lithology, tectonic movement and climate.
Up until now, scholars have done many researches on the geomorphology and formation mechanisms of granite areas and have obtained numerous results. Among those studies, most are carried out from the perspective of morphology, mainly focusing on micro-topography, while little attention is paid to the macro scale basin-range characteristics of granite. For a single rock mass, if there is no obvious in uence of tectonic uplift or subsidence, the basin-range pattern is mainly determined by the difference of resistance to weathering between the rock mass and its surrounding rock. Antiweathering characteristics of different lithologies often vary with diverse climate backgrounds. Under certain circumstances, the characteristics may have completely opposite results. (Cui et al. 2007).
Granite bodies are widely distributed in South China, with variable surrounding rocks and structural characteristics that cause complex basin-range patterns. Understandings on the basin-range formation mechanisms vary. (Li 2008).
In order to deepen the understanding of basin-range formation mechanisms of granite bodies, statistics on the basin-range patterns of the 44 granite bodies in the Hunan Province, where the spatial distributions of granite are relatively simple, were carried out based on the digital elevation data (DEM) and the previous geological survey results. Then, the mechanism of spatial basin-range pattern was revealed based on the combined analysis of tectonic movement and mineral composition.

Introduction to Granite in Hunan Province
The total outcropping area of Granitoids in Hunan Province is about 17,000 km 2 , which formed in different tectonic periods, including Wuling Period (Early Neoproterozoic), Caledonian Period (from Early Silurian to Middle Silurian), Indosinian Period (from Middle Triassic to Late Triassic), and Yanshanian Period (from Middle Jurassic to Late Jurassic). Among them, the second stage of Caledonian (Early Silurian), Indosinian Period (Middle Triassic) and Early Yanshan Period (Middle Jurassic) are the three main peak periods of granite magma intrusion in this area (Fig. 1).
The granites of Wuling Period were found only in Liuyang City, northeast Hunan Province. They were controlled by an EW geological structure of this period and invaded into the strata of Lengjiaxi Group. In Late Silurian Period, the main episode of Caledonian tectonism began, mainly represented by intracontinental orogeny. In Late Caledonian Period, large-scale granitic magmatism occurred in the regions east of Chengbu-Xinhua Fault in the postcollisional environment of weakening compression and stress relaxation (Bai et al. 2006). The rock masses are mainly distributed in some areas surrounding the Late Paleozoic basin in Central Hunan, including Wangyang-Zhuguang Mountain, Penggongmiao, Banshanpu, Baimashan, Miaoershan, Yuechengling, and Jiuweishan.
The Caledonian granite bodies exhibit a widespread and large distribution throughout the study area, most of which were shaped in the second stage of Caledonian Period, and some of which formed compound rock bodies with later intrusive rocks. The Caledonian granites are mainly biotite monzogranite, granodiorite, and syenite granite, of which Baimashan Body, WangyangShan Body, and MiaoerShan Body are the representative granite bodies.

Research Framework and Data
In this paper, the statistics for the basin-range pattern of the 44 granite bodies were carried out rstly based on the digital elevation model (Aster GDEM 30m resolution DEM). Then, the basin-range mechanisms were analyzed from two aspects of tectonic action and weathering resistance. Finally, the erosion levels of basins with different dominant lithology were compared based on the material ux analysis.
In order to support the above study, this research got 394 granite samples and 98 slate samples from all over Hunan Province and sent them to Hunan Province Geological Testing Institute for mineral analysis.
In addition, sediment daily monitoring data from 1961 to 2019 of 10 hydrological stations with different geological background and water monthly quality monitoring data from 2014 to 2018 of 8 stations in Xiangjiang River Basin were collected to carry out material ux analysis (Fig. 1). Both the sediment data and water quality data were the results of the regular monitoring of hydrological stations of Hunan Hydrology and Water Resources Survey Bureau.
In 1987, Zhang et al carried out a large-scale investigation of river water chemistry in the Xiangjiang River Basin of Hunan Province (Zhang et al. 1987), which gained a lot of high-quality water chemistry data. In this paper, a small part of their work is cited to support the material ux analysis.

Analysis of Basin-range Relationship between Granite Mass and Surrounding Rock in Hunan Province
Based on the Aster GDEM data, the topographic map of Hunan Province was drawn and the granite distribution map was nested to obtain the topographic and granite mosaic map of Hunan Province (Fig. 2). Based on Fig. 2, the basin or range properties of all granite bodies were identi ed. Table 1 shows the basin-range type of the main granite bodies.

Analysis of Basin-range Characteristics in Composite Rock Masses
As mentioned above, the basin-range characteristics of North Zhuguangshan rock mass and Baimasi rock mass are unidenti able. In fact, the North Zhuguangshan and Baimasi rock masses are both typical composite rock masses that share basin and range inlay characteristics.
The Northern Zhuguangshan rock mass is located in Luoxiao Mountain in the southwest of Hunan Province, which was formed from Early Silurian Period to Late Jurassic Periods (Fig. 3). The Baimasi rock mass is located in Xuefeng Mountain in the middle of Hunan Province, that was formed from Middle Silurian Perion to Late Jurassic Period (Fig. 4) (Liu et al. 2016).
According to the basin-range relationships among different rocks formed in different geologic periods, the two rock masses both show the same characteristics that, younger rocks are more inclined to develop into ranges than their adjacent older rock masses. Speci cally, all the Silurian rock bodies of the North Zhuguangshan rock masses are basins, while the adjacent Jurassic and Triassic rock bodies are ranges (Fig. 3), and all the Jurassic rock bodies of the Baimasi rock mass are all ranges, while the adjacent older rock bodies are basins. In general, there are always intense magma movements along an orogenic belt with the in uences of the tensile activities of the faults along the belt and the delamination of mantle and lower crust (Wang et al. 1991 After the Caledonian magmatism, some other tectonic movements occurred successively, such as Yanshanian movement and Indosinian movement, and produced many large-scale tectonic systems. The earlier rock bodies may be affected by the later tectonic movement, resulting in some compressional uplifts and staggered faults, which controlled the basin-range evolution processes of some granite masses. For example, the Xupu-Chengbu Fault in the east of the Xuefengshan tectonic belt experienced a strike-slip in the Yanshan Period, which cut off the Caledonian Baimashan rock mass, Indosinian Wawutang rock mass and Indosinian Wutuan rock mass in the north-south direction. For a speci c rock mass, the uplifted part tends to develop into range, such as the Wutuan rock mass.
From the Early Cretaceous to the Paleocene, the tectonic evolution of Hunan Province turned be a new stage characterized with a series of deep-seated extension systems, which produced many horsts and grabens. During this process, some granite bodies were up-lifted and nally developed into ranges, of which Nanyue rock mass is the most representative example.
Since the Miocene Period, the Neotectonics movements have been in uencing the surface morphology of most areas in Hunan Province. In this stage, the crustal uplifts in eastern, southern, western and north-western Hunan are obvious and continuous, while the Dongting Lake area in the northeast has been intermittent subsidence (Bo et al. 2011). The crustal uplift accelerated the denudation of overlying strata of granite rock masses and exposed some deep granite bodies to the surface (Bell 2016).

Comparison of mineral compositions between granite rocks and their surrounding rocks
According to the previous statistics, the surrounding rocks of the granite basins in Hunan Province are mainly slate, and of the granite ranges are mainly carbonate rocks. Hunan has the subtropical monsoon climate with relatively high temperature and large rainfall and chemical weathering is the dominant weathering type. The carbonate areas always present as basins just because of the higher solubility of calcite and dolomite than granite minerals (Pye 1986;Cui et al. 2007). It can also be proved that the difference of mineral contents between granite and slate determines the basin-range relationship between them.
The granites in Hunan Province are mainly monzonite, with less amount of granodiorite. The average mineral compositions of granitic bodies in different geological ages can be obtained based on statistical analysis of the mineral compositions of 394 granitic samples ( Table 2). Table 2 shows that, the average contents of K-feldspar, plagioclase, quartz and mica for 394 samples are 31.1%, 33.1%, 29.3% and 5.7% respectively, and the mineral contents of rock masses in different geological ages are obviously different. The average contents of plagioclase and biotite in the Silurian Period and Middle Triassic Period are signi cantly higher than the later geological ages, while the average contents of K-feldspar and quartz are lower than the later ages. Goldish and Carroll (Goldish 1938; Carroll 2012) summarized the mineral-stability series in weathering, namely quartz > muscovite > K-feldspar > plagioclase > biotite > hornblende > pyroxene > olivine. According to this series, the rock bodies of the Silurian Period and the Middle Triassic Period is more likely to be weathered than the later rock bodies, that is the main reason why the younger rock bodies are more likely to develop into ranges in a composite rock mass. The slates in Hunan Province mainly formed in the Wuling Period and Caledonian Period under the in uence of regional tectonic movements, which are distributed in the strata of the Proterozoic Lengjiaxi Group, the Proterozoic Banxi Group, the Nanhua Period and the Silurian Period (Zhang et al. 1987). Statistical results of the average mineral contents of different types of slate were given in Table 3. Table 3 shows that, sericite and quartz are dominate minerals for the main types of slate. Except for carbonaceous slate, the content of sericite and quartz of other types of slate accounts for more than 80% at least.
As a ne-grained type of muscovite, sericite is the product of low and medium temperature alterations or shallow metamorphisms, and has stable chemical properties (Yang et al. 2006). Experiencing more tectonic movements accounts for the higher fracture rate of the slates in Hunan Province which formed ahead of Silurian, whereas their mineral compositions are more resistant to weathering.

Comparative analysis of soil in ltration conditions between granite and slate
In those areas dominated by chemical weathering, rainfall in ltration plays an essential role in the weathering process. The more rainfall in ltrates into the soil, the more opportunities for water-rock interaction and accordingly the faster the weathering.
Determined by the unique characteristics of mineral compositions, weathered soil of granite is characterized by high sand and gravel content, loose soil, and large permeability (Nesse 2012). According to the statistical result completed by Zheng and Bennett (2002), the permeability coe cient of weathered granite soils is generally in the range of 0.25 ~ 2.50m/d.
Slate is mainly formed by light metamorphism of mudstone, argillaceous siltstone or tuff, and the weathering products on its surface are mostly clay or loam. The permeability coe cient of this soil is generally between 0.001 and 0.01 m/d (Zhang et al. 2009), which is much lower than that of granite weathered soil. As a result, it may be argued that the rainfall in ltration in granite area is greater than that in slate area, which is another important reason why the weathering rate of granite is larger than slate in hot and humid climate.

Comparative analysis of material ux analysis
For a closed basin, material ux of the export can roughly re ect the average weathering rate of the basin if without considering the impact of human activities (Gurumurthy et al. 2012). The average sediment transport modulus and chemical ux modulus of each catchment area determined by related hydrological station were calculated based on the collected sediment and water quality data and the results were showed in Table 4 and Table 5.
It can be concluded from Table 4 and Table 5, that the sediment transport modulus and chemical ux modulus of all basins are closely related to the dominant lithology.  respectively, which are well consistent with the results of this paper.
Based on the above analysis of the sediment transport and chemical ux modulus, the overall material ux without considering water of the basins can be roughly compared. In the slate basin, the sediment transport modulus and chemical ux modulus are both small, that means the basin's weathering is also at a low level. Although granite basins present low hydrochemical ux modulus, the total material ux modulus are much higher than slate basins for its highest sediment transport modulus. Controlled by the characteristics of easy-dissolved, the hydrochemical ux modulus of carbonate rock basin is much higher than that of other types of basins, which makes its total material ux the highest, though it has lower sediment transport ux modulus.

Conclusions
In this paper, a statistical study on the basin-range relations between the granites and their surrounding rocks in Hunan Province was carried out, and the formation mechanisms were analyzed from two main aspects of tectonic evolution and the differences of anti-weathering between the granites and their surrounding rocks. The result was veri ed by the analysis of material ux.
Among the 44 granitoids in Hunan Province, 28 are basins, 14 are ranges, and only 2 cannot be directly classi ed. According to the statistical results of their surrounding rocks, the basin-range properties of granite bodies are closely related to the lithology of surrounding rocks. Among the 28 granite basins, 24 are surrounded by slate, 3 by sandstone, and only 1 by glutenite, while 11 of 14 ranges are surrounded by carbonate rocks.
Tectonic movements determined the large-scale basin-range pattern of granites in Hunan Province. Controlled by the large-scale tectonic framework, some granite concentrated zones are formed along some mountain ranges, including Xufeng Mountain, Luoxiao Mountain, and Nanling Mountain. The earlier rock bodies may be affected by the later tectonic movement, resulting in some compressional uplifts and staggered faults, which controlled the basin-range evolution processes of some granite masses, such as the Wutuan Rock Mass and the Nanyue Rock Mass.
The difference of resistance to weathering between granite and surrounding rock is a controlling factor for the development of granite basin or range. Because of the high solubility of carbonate minerals, the weathering speed in carbonate area is the fastest, so the granite rock areas always present as ranges when their surrounding rocks are carbonate rocks. The minerals of slate rocks in Hunan Province have higher resistance to weathering than granite minerals. In addition, the surface soil in granite area is more conducive to rainfall in ltration than slate area, which means that the granite has more chances of weathering. Controlled by the above two factors, the weathering rate of the granite areas in Hunan Province is higher than that of the slate areas, so that when the surrounding rocks are slate rock, the granite areas always present as basins. The differences of resistance to weathering between different types of rock basins are veri ed by the analysis results of sediment transport modulus and hydrochemical modulus. Figure 2 Nested map of granite and topography in Hunan Province.

Figure 3
Basin-range framework of the North-Zhuguangshan rock mass. Basin-range framework of the Baimasi rock mass.