Genesis And Tectonic Significance Of Middle Permian-Late Triassic Magmatic Rocks And Discussion On Their Relationship With Hot Dry Rocks Around The Gonghe Geothermal Basin,Western Qinling

The Gonghe geothermal basin in the West Qinling is located in the northeast margin of the Qinghai-Tibet Plateau,which is an important junctional area of the superposition and conversion of several blocks of the West Qinling,South Qilian,East Kunlun,Qaidam and Oulongbuk.Its special tectonic location makes it an ideal site to decipher the transformation of the tectonic systems between Proto-and Paleo-Tethys,the convergence of micro-blocks,the geological setting of multi-stage ore deposits and new energy resources（such as high-temperature hot dry rocks）.However,there are still some scientific debates,including the crystallized age,magmatic source area,and regional tectonic-magmatic thermal evolution.In addition,with the exploration of hot dry rock resources in recent years,some hot dry rocks with a high temperature of236℃has been ascertained at a depth of 3705 m beneath the Gonghe geothermal basin.This is the first time to drill the dry hot rocks with the characteristics of the shallowest and hottest in China,realizing a major breakthrough in exploration of dry hot rocks.The carrier of dry hot rocks in the basin is composed of Triassic granite,so what roles do granites play in the genesis of dry hot rocks resources?Is it a caprock,or a reservoir,or a heat source?Besides,the geological background of the formation of hot dry rocks is still the most concerned topics in the Gonghe geothermal basin.Based on the systematic collection from previous research data,granites in the southern margin of the Gonghe basin and dioritic to felsic rocks in the Zongwulong-Nanshan tectonic belt have been taken as the research objects.In this contribution,the data covering the major and trace elements,zircon U-Pb chronology,in-situ Hf isotopes of zircon and radioactive heat generation rate of rocks have been presented to constrain the genesis and tectonic significance of Middle Permian to Late Triassic magmatic rocks and their relationship with the formation of hot dry rocks in the Gonghe geothermal basin.This study has mainly achieved the main conclusions as follows:1)Field geological characteristics,crystallized age,geochemical characteristics,magmatic source area and geodynamic setting of the subducted granite in the southern margin of the Gonghe geothermal basin has been identified.Field investigation shows that there are a large number of acid intrusions distributed in the southern margin of the Gonghe basin,which are composed of granodiorite,monzogranite and syenogranite with different grains in size.Zircon U-Pb dating shows that granodiorite was formed at ca.251-249 Ma,monzogranite at ca.251-246 Ma and syenogranite at ca.250-243 Ma.Zircon Hf isotope analysis show thatεHf（t）values of ca.251-243 Ma granite range from-7.10 to+2.22 and their second-stage crustal Hf model ages（TDMC）belong to Late Paleoproterozoic to Early Mesoproterozoic.The whole rock geochemical analysis shows that ca.251-243 Ma granite belongs to the weak peraluminous and high-K calc-alkaline rock series.They have similar distribution curves of rare earth elements,showing the right-leaning pattern of enrichment of LREE and relative deficit of HREE.Most of them are strongly to moderately deficient Eu anomalies with Sr/Y values of 0.68-42.29 and Nb/Ta values of 0.30-18.00,belonging to the normal arc magma series.The acid rocks in the southern margin of the basin are I-type granite with varying degrees of differentiation.The source area may be partial melting of the middle-lower crust amphibolite in the Late Paleoproterozoic-Early mesoproterozoic,with the addition of a small amount of mantle material.The differentiations of plagioclase,potassium feldspar,apatite,monazite and/or epidote,as well as Ti-bearing minerals,play an important role in the magmatic evolution process.The granite in the south margin of Gonghe basin was formed in the active continental margin arc related to the subduction of the Paleo-A’nimaque oceanic plate.2)Late Permian-Late Triassic acidic magmatism in the Zongwulong-Nanshan tectonic belt has been ascertained.Field investigation shows that the Zongwulong-Nanshan tectonic belt is mainly composed of medium-grained granodiorite,medium to coarse-grained porphyritic monzogranite,medium to fine-grained syenogranite and fine-grained felsic dikes.The rock association of the outcrop body is consistent with that of the buried magnetic granite body（hot dry rock reservoir）.Zircon U-Pb dating shows that granodiorite was formed at ca.255-248 Ma,monzogranite at ca.251-240 Ma,syenogranite at ca.251 Ma.This is consistent with the crystallized age of hot dry rock reservoir reported by predecessors（granodiorite ca.244-242 Ma,monzogranite ca.245-236 Ma）.The fine-grained felsic dikes formed at ca.230 Ma,which is the Late Triassic.The Hf isotope analysis shows that theεHf（t）values of ca.255-240 Ma granites vary from-5.97 to+2.04 and their second-stage Hf model age（TDMC）belongs to the Mesoproterozoic.TheεHf（t）values of ca.230 Ma felsic dikes range from-6.90 to-1.63 and their TDMC belongs to Late Paleoproterozoic to Early Mesoproterozoic.The whole rock geochemical analysis shows that the ca.255-240 Ma granites and hot dry rock reservoirs belong to the metaluminous-weak peraluminous,high-K calc-alkaline rock series.The ca.230 Ma felsic dikes belong to the alkali-silica-rich,peraluminous shohshonite series.The ca.255-240 Ma granites and hot dry rock reservoirs may be derived from partial melting of the juvenile lower crust in the Paleoproterozoic.The ca.230 Ma peraluminous felsic dikes may be derived from partial melting of the middle to upper crust pelites from the Late Paleoproterozoic to Early Mesoproterozoic.The former may be related to the subduction of the Paleo-Zongwulong Ocean,while the latter may be related to the collision to post-collision stage of the ocean.With the continuous evolution of the magma,the more acidic and highly differentiated magma rich in alkali and silicon,and rich in volatiles was gradually formed.Ca.230Ma felsic dikes are the result of the fissure emplacement produced by the condensation and contraction of the ca.255-240Ma granites.3)The crust-mantle magmatism related to the subduction system of the paleo oceanic crust in the Zongwulong-Nanshan tectonic belt has been revealed.Field investigation and petrographic observation show that diorite is dark gray and fine-grained texture,which is mainly composed of plagioclase and amphibole and a small amount of clinopyroxene and biotite.Plagioclase is idiomorphic to sub-euhedral texture with the twin-striation.The euhedral plagioclase phenocrysts are‘polyporphyritic aggregates’or‘chain structures’.Hornblende is idiomorphic to sub-euhedral texture.Quartz diorite is light gray,medium to fine-grained,massive,mainly composed of plagioclase,quartz,hornblende,biotite and K-feldspar.Most mineral crystals exhibit idiomorphic to sub-euhedral granular structure.In addition,euhedral plagioclase crystals show zonation of composition,and there are also closely packed plagioclase phenocrysts,which appear as‘polyporphyritic aggregates’.Hornblende appears as a subeuhedral-euhedral grain with a‘chain structure’.The closely packed quartz is presented as‘glomeroperphyritic aggregates’.Zircon U-Pb dating shows that diorite crystallized at ca.264-255 Ma and quartz diorite crystallized at ca.261-254 Ma.The Hf isotope analysis shows thatεHf（t）values of diorite range from-4.19 to+8.89,corresponding to crustal Hf model ages of 626-1140 Ma.TheεHf（t）values of quartz diorite vary from-4.05 to+4.67,and the crustal Hf model age is737-1098 Ma.Geochemical analysis of the whole rock indicates that the diorite belongs to the metaluminous,calc-alkaline to high-K calc-alkaline series.Mg O is relatively high（3.09-7.11%）and the calculated average Mg#is 54,belonging to high magnesium diorite.The distribution curve of REEs is relatively uniform,showing a right-leaning pattern of enrichment of LREE and relative deficit of HREE,and a weak deficit Eu anomaly（Eu/Eu*=0.78-1.02）.They were enriched in LILEs and Pb,but depleted in HFSEs and Sr,P and Ba.The values of Nb/Ta and Sr/Y were 10.07-14.96and 11.90-23.05,respectively.Quartz diorite belongs to the metaluminous-weakly peraluminous and high-K calc-alkaline series.The distribution curve of REEs is relatively uniform,showing a right-leaning pattern of enrichment of LREE and relative deficit of HREE,and a moderate-weak Eu deficit（Eu/Eu*=0.42-0.83）.LILEs and Pb were enriched;while HFSEs and Sr,P,Ba were depleted;Nb/Ta values are from 10.75 to 14.91,and Sr/Y values are from 4.44 to 19.34.The diorite may be partial melting of the lithospheric mantle metasomatized by the melt of previous subduction-related sediments.Quartz diorite derived from partial melting of the lithospheric mantle,which is metasomatized by sediment melt and water-bearing fluid associated with previous subduction.Both the fractional crystallization of olivine,chromite,monoclinic pyroxene and plagioclase and the accumulation of amphibole,plagioclase and quartz play an important role in forming the diorites.The diorite and quartz diorite have arc magmatic properties and are formed in an active continental arc margin environment.（4）The temporal and spatial variations and geological significances of Late Paleozoic to Early Mesozoic magmatic rocks in the Gonghe basin have been identified.The magmatism is intensive in the southern margin of the Gonghe Basin,which is characterized by acid magmas.The crystallized ages tend to be younger from the northern Qieji,to the central Daheba,and then to the southern Ningxiu-Heri area.However,the rock assemblages have not changed too much,mainly granodiorite+monzogranite+syenite.The potassium contents in magma are relatively high,inplying the rocks belong to the high-K calc-alkaline series.While the aluminum contents are low and they are metaluminous to weak peraluminous.The acid plutons in the south margin are high-K calc-alkaline I type granite with different degrees of differentiation.The geological evolution in the southern margin of the Gonghe basin is similar to that of the eastern section of the East Kunlun,and both are probably the products of one tectonic cycle that is related to the northern subduction of the Paleo-A’nimaque Ocean during the Late Paleozoic to Early Mesozoic.Actually,the subduction of the Paleo-A’nimaque oceanic crust began at ca.278 Ma,and it has undergone a continuous subduction history of 40 million years.As the subduction went on,the subduction plates broke off and a large volume of mantle-derived magma intruded into the lower crust to form juvenile crust.The formation of the juvenile crust caused by the addition of mantle-derived magma is an important way for the growth and evolution of the Early Mesozoic continental crust in the southern margin of the Gonghe Basin.The magmatic activities in the Zongwulong-Nanshan tectonic belt show a young trend from west to east.The magmatism in the belt can be divided into two stages,namely ca.271-236 Ma and ca.235-203 Ma.The early magmatic activity（ca.271-236 Ma）is correlated to the subduction stage of the paleo oceanic crust;while the late magmatic activity（ca.235-203 Ma）is the collusion to post-collusion stage between the Paleo-Zongwulong oceanic crust and West Qinling block.The tectonic belt has experienced about 35 million years of oceanic subduction history,which began in the Late Permian（ca.271 Ma）and lasted until the Middle Triassic（ca.236Ma）.The long-lived subduction of the oceanic crust resulted in the continuous upwelling of metasomatic mantle-derived magma,which is an important way of crustal growth during the Late Paleozoic to Early Mesozoic in the Zongwulong-Nanshan tectonic belt.On the spatial scale,under the subduction background,the Zongwulong-Nanshan tectonic belt went through local melting at different depth levels（metasomatism mantle source region and junvenile mafic lower crust source region）.On the time scale,the mantle source area of the Zongwulong-Nanshan tectonic belt went through metasomatism related to subduction fluids（ca.271 Ma）,sediment melts（ca.264 Ma）and plate melts（ca.249 Ma）.Thus,there is a coevolutional relationship between arc magmatism and mantle metasomatism during Late Paleozoic and Early Mesozoic in the Zongwulong-Nanshan tectonic belt.By comparing the evolutional characteristics of regional ocean basins in Late Paleozoic to Early Mesozoic,the Paleo-A’nimaque Ocean in the southern margin and the Paleo-Zongwulong Ocean in the northern margin obviously went through completely different geological processes,belonging to two independent evolutionary ocean basins.Both of them are branches of the Paleo-Tethys Ocean.（5）The genetic relationship between granites and hot dry rocks in the Gonghe geothermal basin has been established.Vertically,the Gonghe basin can be divided into four layers,namely Quaternary sediments,Paleo-Neogene mudstone,siltstone,interbedded sandstone and conglomerate,Triassic metamorphic rock and hidden magnetic granite thermal reservoir.The buried magnetic granites beneath the basin and granitic rocks in the outcrop may be connected in depth,and they may share a common magma chamber.The average radioactive heat generation rates of medium to coarse-grained granites in Late Permian to Middle Triassic are between 1.85 and 6.85μW/m³,indicating that the rocks are moderate to high heat production.During the evolution of the Paleo-Zongwulong Ocean,the basin stayed in a subduction setting for a long time.On the spatial scale,the basin experienced partial melting at different depth levels.On a time scale,the deep mantle source area of the basin has undergone metasomatism related to subduction fluid,sediment melt and plate melt.At the same time,a large volume of dioritic to felsic magmas frequently invaded in Gonghe basin.The longer the cooling crystallization process,the more conducive to long-term extraction,it will result in continuous accumulation and enrichment of U and Th elements in the rocks.In addition,since ca.65 Ma,the strong collusion-extrusion has occurred in the northern margin of the Qinghai-Tibet Plateau.With the thickening of continental crust,the thickness of the radioactive element enriched layer in the crust increased simultaneously,resulting in the enhanced heat contribution of the radioactive heat generation rate in the crust,which increased the crustal heat flow and the surface heat flow.The thickened medium to coarse-grained granite layer can provide a stable heat source base（about 11-18%）for hot dry rocks geothermal resources in Gonghe basin.The partial melting layer in the crust with a depth of 15-35 km,which formed at ca.9.7-1.8 Ma and was related to magmatism,can provide an additional favorable heat source for the formation of hot dry rocks.Fine-grained densely massive felsic dikes（ca.230 Ma）have extremely low rock porosity and act as an important"thermal insulation"layer to resist heat dissipation.The Cenozoic argillaceous sedimentary cover with a large thickness at the top of the basin has very low thermal conductivity and thermal diffusion coefficient,which plays the role of secondary heat accumulation.The buried large fault zone beneath the basin transects the geothermal field,forming a local heat transport system,through which the deep hydrothermal fluid or melt is transported to the shallow granite thermal reservoir.The intersection of two or more hidden deep faults is more conducive to the formation of hot dry rocks.The medium to coarse-grained porphyritic granites formed in the Late Permian to Middle Triassic have high thermal conductivity（7.5 times that of the Cenozoic pelitic rocks）,thermal diffusivity(up to 1.87×10^-6 m²/s),and greater porosity（granite porosity up to 3.6%at15 km depth）.It also plays an important role in the transfer of deep high temperature heat flow to shallow thermal reservoir.Finally,according to the model,we consider that complex rock itrusions with multiple stages of magmatic emplacement may be an important target for dry-hot rock exploration,especially the porphyricite-bearing syenite granite,which provides basic data for the implementation of EGS dry-hot rock geothermal resources exploitation.