The History And Significance Of The Uplift And Exhumation Of Laojunshan Late Cretaceous Granite Body In Malipo,SE Yunnan

Laojunshan Late Cretaceous granite body is a good research object to reveal the uplift and exhumation history of Malipo district since Cenozoic.In this paper,we use zircon and apatite fission track technology,take age-elevation relationship method?mineral pair method and thermal history modelling method to analyze the tectonic-thermal evolution history of Laojunshan granite body.Based on field study of structural geomorphology,the space-time characteristics of rock uplift and its dynamic background are discussed.In addition,the effect of granite body uplift on the preservation of Dulong tin-zinc polymetallic deposit is also discussed after field geological survey.The main points are as follows:(1)The process of uplift and exhumation of Laojunshan granite body is complicated,and the different chemical composition of apatite and severe radiation damage in zircon,cause the fission track age gained from experiment cannot represent cooling age.If the age-elevation method or the mineral pair method is used alone to analyze fission track age data,the history of cooling and uplift obtained will be inaccurate and incomplete.Therefore,the fission track research in Laojunshan granite body should preferably depend on thermal history modelling method,and supplemented by age-elevation method and mineral pair method.(2)The comprehensive analysis shows that the Laojunshan granite body has undergone a complicated cooling history since the Late Cretaceous,which has the characteristics of multi-period temporality and local spatial differences.Specifically,four stages of rapid cooling events can be divided: the first period of 75-42Ma;the second period of 31-20Ma;the third period of 15-10Ma;the fourth period of 2-0Ma.The first and second stages of rapid cooling were collectively recorded by the entire Laojunshan granite rock,suggesting the overall uplift and exhumation of the granite body.The third stage mainly occurred in the northeastern part of the granite body,while the sample from northern edge of the granite body simulated the fourth stage out.The latter two stages reflected the transition of the uplift from the overall to the local difference.(3)All four stages of uplift and exhumation in Laojunshan district are consistent with the dynamic background of regional tectonic evolution.The uplifting and exhumation event of 75-42 Ma may be related to intraplate extension which happened in the South China plate during the Late Mesozoic to Early Cenozoic,and a later period(53-42Ma)uplift can also be interpreted as a result of remote response related to the initial collision of India-Eurasia at 55 Ma.The uplift and exhumation at 31-20 Ma and 15-10 Ma respectively corresponded to the successive extrusion of the Indosinian block and the Sichuan-Yunnan block during the long-term lateral escape of the southeastern margin of the Qinghai-Tibet Plateau.The latest 2-0Ma exhumation reflects the reactivation of the faults in the north side of the granite body since the Pleistocene.(4)The field structural survey shows that there are at least two stages' structural deformations in the southern region corresponding to the northern Laojunshan granite body uplift.The early extensional sliding and late normal faulting respectively played a role in the preservation and destruction of the Dulong tin-zinc polymetallic deposit.