The formation and evolution model and development stages of granite landforms

The micro-landform landscape cluster of Sanqingshan Granite is a rare granite geological and landform natural relic area in the world. It has clear characteristics and systematic exposures. It records and preserves the history of crustal formation and evolution since the Mesozoic and Cenozoic, especially the more systematic and It completely demonstrates the formation and evolution rules of the geological and landforms of the Sanqingshan granite, and reveals the main process of diagenesis → mountain formation → landscape formation of the Sanqingshan granite (Figure 6.5).

Figure 6.5 Schematic diagram of the Sanqing Mountain diagenesis and mountain formation model

(1) Formation stage of granite rock mass: During the Mesozoic period of earth evolution and development, the Sanqing Mountain area responded to the subduction of the ancient Pacific plate and During the lithospheric thinning event in the east, intracontinental deep subduction-type magmatism occurred in the ancient plate suture zone, and large-scale acidic magma intrusion occurred. After condensation and crystallization, a typical granite field was formed in the Sanqingshan area.

Although different scholars have different views on the magnitude, specific time, spatial scope, and mechanism of Mesozoic lithosphere thinning in eastern my country (Wu Fuyuan et al., 2008), lithospheric thinning has become a scientific The proposition is generally accepted. Combining the regional geological development history and zircon U-Pb dating results, it is speculated that the formation process of the Sanqingshan rock mass is as follows.

Affected by the subduction of the Pacific Plate, this area is in a state of compression, and the source area of ??crust-mantle mixing partially melts to form ore-bearing porphyries, such as the Dexing porphyry copper deposit and Jinshan gold deposit. However, after a long period of compression, the stress was released and the area was in a state of tension during the Cretaceous, resulting in the formation of a series of extensional rift basins along the Shaoxing-Yingtan fault zone in the south and the Liyang-Tunxi-Dexing fault zone in the north. , and caused the partial melting of crustal metamearly rocks to form granitic magma at a distance of 10km from the surface. The thinning of the lithosphere provided space for the magma to emplace, and it invaded the axis of the Huaiyushan complex anticline under the action of horizontal tension. , located about 6km from the surface, the slow and long-term crystallization formed the coarse and medium-grained biotite potassium feldspar granite (i.e. the central phase) in the park; around 123Ma ago, the earth's crust partially melted again, and the magma followed the original channel It ascended and invaded the early medium-coarse-grained granite, and condensed to form medium-fine-grained biotite K-feldspar granite (i.e., transitional phase); around 116 Ma ago, under a similar extensional environment, the crust partially melted twice, but the melt The scale is not large, and it cools quickly to form fine-grained (micro-grained) biotite monzonite (i.e., the smallest edge phase and supplementary phase). Erosion of uplifted crust and accumulation in subsidence basins proceed simultaneously. Calculated based on the emplacement depth of 6 km, that is to say, after the 6000 m thick strata above the Sanqingshan granite body were denuded, they accumulated in the fault basins on both sides of the block, forming the extremely thick Early Cretaceous Zhoujiadian Formation and Late Cretaceous Zhoujiadian Formation. Cretaceous Nanxiong Formation (Zu Apingping, 2004; Yu Xinqi et al., 2005).

(2) The formation stage of the granite mountain: Against the background of the uplift of the Huaiyu Mountain terrane, the Sanqing Mountain granite body also gradually exposed the surface as the uplift and erosion progressed, and moved in the northeast and north directions. Under the control of three faults running northeast and northwest, Sanqing Mountain is cut into a typical triangular fault block mountain, and neotectonic movements have caused it to further rise significantly. The unique "uplift on uplift" structure has created the mountainous landform pattern of Sanqing Mountain represented by peaks such as Yujing Peak, Yuxu Peak and Yuhua Peak.

(3) Granite landscape formation stage: Under the control of joints and fissures, the granite mountain has been carved by geological processes such as weathering and erosion, freezing collapse, and water erosion, forming today's peak walls, peak clusters, and peak pillars. The unique granite peak forest landform combination.

The formation of peak walls and peak clusters: Based on the formation of triangular fault block mountains, controlled by vertical joints in two directions in the Sanqingshan granite body, the granite body was cut into a checkerboard shape. Under the continuous action of , some rock masses form peak wall landscapes along large-scale fracture joint zones, and some rock masses form peak cluster landscapes.

The formation stage of the peak pillar: on the basis of the peak cluster and peak wall landscape, as the external geological effects such as water erosion, collapse, weathering and erosion continue, the canyon widens and the peak wall narrows. The peak walls are gradually cut into ruins to form peak pillars; the valleys around the peak clusters are cut deeper and deeper, gradually forming peak pillars, and some also form smaller stone cones.

The formation stage of the modeling stone: after the peak pillar is formed, the near-horizontal joints cut the pillar into two sections of rock. Some are formed into spherical bodies by spherical weathering, and some are formed by collapse and water erosion. Forming various shapes of modeling stones.

Geomorphologist Twidale (1982, 1986, 1993) used the theory of granite "double-sided weathering crust" (Budel, 1957, 1965) to explain the formation and exposure process of granite eggs.

In tropical and subtropical areas, granite forms a thick layer of weathering crust. The upper part is a deeply weathered weathering crust, and the lower part is an undulating bedrock surface composed of fresh rock layers, which is the weathering front (Figure 6.6).

Figure 6.6 Schematic diagram of the formation of granite "stone eggs"

(Modified after Cui Zhijiu et al., 2007)

The section of the granite weathering crust generally has the following features from the surface downwards Pu Qingyu (2007) divided the zones into six layers: soil layer → red soil layer → sand layer → gravel layer → rubble layer → bedrock; Cui Zhijiu (2007) divided five zones: extremely weathered zone → Strong weathering zone → Moderate weathering zone → Weak weathering zone → Fresh bedrock zone.

The meanings of the two division methods are the same, and the five zones correspond to the last five layers of the six layers. Some granite stones with smaller particle sizes remain in the moderate weathering zone, filled with sand. The weakly weathered zone, also called the upper bedrock layer or stone egg layer, basically preserves the original structure of granite. The granite blocks are separated from each other and spherically weathered to form stone eggs. As weathering and erosion progress, the weathering front continues to advance toward the lower part, and the ground is gradually eroded lower. The stone layer (stone egg layer), gravel layer, sand layer, and red soil layer are constantly replaced in an orderly manner. If the denudation speed of the weathering crust exceeds the downward movement speed of the front, the laterite layer, sand layer and gravel layer of the weathering crust will disappear one after another, and the remnants of the stone layer or fresh bedrock will be exposed.

Stone eggs are mainly exposed near Sanqing Palace and "Calabash Stone" in Sanqing Mountain. It is speculated that during these two ancient primeval periods, two ancient weathering crusts developed on the ground. During the uplift of the mountain, these The ancient weathering crust has basically been eroded away, and only the remains of ancient weathering crust remain near Sanqing Palace.

The classification of geological periods according to W.M. Davis's erosional cycle theory is a generally accepted classic model: that is, it is divided into three main stages: infancy, adulthood and old age, and several sub-stages or transitional stages. , and then the "landscape rejuvenated" and started a new cycle. Landform development stages are mainly judged based on landform morphology. Granite landforms can be divided into four stages (Cui Zhijiu et al., 2007), see Table 6.3.

Table 6.3 Development stages of granite landforms and their corresponding landform signs

Even in the same mountain, multiple landform types often coexist, which reflects the spatial evolution law of landforms. This is the case in Sanqing Mountain. The key lies in the development stage represented by the main landform signs. Sanqing Mountain is a typical peak forest landform area characterized by a combination of granite cone-shaped peaks and dense peak pillars. It is in the stages of late childhood and early adulthood, while Huangshan is in its prime. This is also consistent with the evidence of geological dating, and the last intrusion period of the Huangshan rock mass was (125±1.5) Ma (Liu Xiyuan et al., 2005), which is consistent with the formation time of the transitional phase of the Sanqingshan rock mass (123Ma±2.2Ma ) are basically at the same time. It is obvious that the time of the replenishment period of the Sanqingshan Mountains (115.6Ma±2.0Ma) is slightly later than the time of the last phase of the Huangshan rock mass. It can be expected that the "tomorrow" of the Sanqingshan Mountains will be the "today" of the Huangshan Mountains.