| As a part of A-type granite belt in the eastern China, Laoshan Mountain granite is not only an upward development of many natural-mortar, there are also a large number of lateral opening of the bubble-like cave. Different researchers used different names to explain the nature-mortar in Laoshan, such as "glacial pothole","weathering pit".Song Zhaojun et al found Laoshan primary granite caves that are large bubble-like cave and miarolitic, and put forward the "natural-mortar and bubble-like cave homology". However, some people think that these caves are "Karst cave", it was also suspected of "sea-erosion caves".Through a lot of on the spot investigation of Laoshan large bubble-like cave fieldwork, and bubble-like cave in Laoshan study and natural-mortar on sampling, this paper will further demonstrate the reason of formation about large bubble-like cave.In this paper, the micro-morphological characteristics of large bubble-like cave walls are studied through macroscopic (visual observation) and microscopic (stereoscopic and scanning electron microscope(SEM)),and contrast the surface micro-morphological characteristics of rocks which are of different motive genesis (aqueous effect, aeolian effect and artificial effect). The results show that there are obvious differences between the large bubble-like cave wall and other types of rock surface: First, the surface of the bubble-like cave wall is relatively smooth and flat without obvious scratches, The quartz and feldspar mineral particles are basically the same height, no ups and downs. Second, there are only a small amount of cracks in the surface of the bubble-cave wall, and most of the mineral crystal form is relatively complete, Quartz and feldspar minerals are closely linked without cracks existing in contact parts , and the mineral boundary is obvious, from which can clearly distinguish the shape of quartz and feldspar minerals.Third, the alteration degree of Laoshan large bubble-like cave wall’s rock surface is low, only in the quartz and feldspar mineral boundary is there a small amount of scaly aggregates of sericite.This paper uses X-ray diffraction (XRD) to analysis rocks and minerals on the walls and outside surface of the large Laoshan bubble cave and with the inner surface of stone mortar of Lingqi Peak, therefore analysing the difference of illite content and quartz-feldspar ratio between various rocks’ samples can contrast the weathering degree of samples. As a result, there is no illite in rock mineral composition on the walls of the bubble-like cave, and the average of its quartz-feldspar ratio is the lowest, what’s more, each rock’s quartz-feldspar ratio on the walls of the bubble-like cave is less than that on the bubble-like cave outside or the inner surface of mortar, so which shows that the weathering degree of rocks on the walls of the bubble-like cave is relatively low and on the verge of fresh rocks, and the rocks are not eroded by the external force. Meanwhile, this paper also makes use of X-ray fluorescence(XRF) to make an analysis of main elements of rocks on the wall of Laoshan large bubble-like cave, those outside the cave and those in the mortar of Lingqi Peak, then according to the measurement results, calculating separately index of weathering of each sample, including Chemical Index of Alteration (CIA), Weathering Index of Granite(WIG), Weathering Index of Colman (WIC)and Weathering Index of Parker(WIP), thus contrasting the weathering degree of various types of rocks and the external force it may be subjected to through the relationship between index of weathering and the weathering degree. And the results show that the weathering degree of rocks on the walls of the bubble-like cave is at the low stage of weathering, that is,the lowest, in addition, there’s been no trace of hydrodynamic action on the walls; while the weathering degree of rocks in large Laoshan bubble-like cave outside is higher than that on the walls, in which rocks’weathering degree with the inner surface of mortar is the highest and it has suffered severe hydrodynamic action.In conclusion, the contrast between the surface features of rock caused by different powers in laoshan area and chemical characteristics of rock mineral shows that weathering degree of the large bubble-like cave wall is relatively low which is similar to the fresh rocks, Beside, the degree of the rocks on the wall of Laoshan large bubble-like cave suffered from weathering is lower than the rocks out of the cave and in the mortar which belongs to lower weathering, the cave was not caused by weathering, but by the rocks were suffered from weak weathering when they were exposed in the air; According to this Laoshan large bubble-like cave were formed without any external power which is neither "karst cave"、"sea-erosion caves" or "deflation cave". So we can testify that is classified in natural cave. This conclusion provides the evidence for the fact that bubble-like cave is granite primary cave. |
