1. Metamorphism of ophiolite melange belt in central Dongkun.
In space, Buqingshan in Cuona, Hubei Province in the east, Xialaguole in Lake Arak with a longitude of1∶ 250,000 in the west and Tage in Chulak, north of Bukadaban Peak, are covered by Cenozoic sediments around Lake Aqi Kekule. The western end is intermittently exposed near Chaoyanggou and cut by Altun fault zone to the west, resulting in the sporadic preservation of this metamorphic belt in West Kunlun. Metamorphic geological bodies include Wanbaogou Group and Nachitai Group from the middle Neoproterozoic to the Lower Paleozoic, as well as structural blocks of Xiaomiao Group and Baishahe Group with mixed structures. In winter, the metamorphic rocks in the area of Qian 'gennager, north of Lake Misna, were studied in detail.
(1) rock mineral characteristics
Wanbaogou rock group and Nachitai rock group distributed in the areas of Qiangegegouer and Dariwanakagou include biotite quartz schist, muscovite quartz schist, garnet biotite-epidote schist with plagioclase residual spots, epidote schist, tetrahedrite-black Yunyang schist and uplift epidote schist (and corresponding structural schist), chlorite epidote schist and actinolite. The characteristic metamorphic minerals are garnet, amphibole, plagioclase, mica and chlorite. As shown in Figure 4-8, the composition zoning of garnet profile belongs to eclogite species. The asymmetry of garnet composition profile is consistent with the optical band of garnet, which can be divided into two sections. The early XSpe section is bell-shaped, which is the product of normal metamorphic growth and corresponds to the core of garnet-rich inclusions. In the later stage, XSpe rebounded, reflecting that some areas were readjusted due to degradation interference, but the composition profile of calcium-aluminum garnet was not disturbed, and the ratio of Mg/(Mg+Fe) generally increased, corresponding to the edge of bad inclusions in garnet. In garnet albite amphibolite schist in exotic rocks, the garnet wrapped in albite is XAlm=0.762, XGRO = 0. 184, which is mainly composed of almandine and calc-almandine molecules, and the calcium enrichment of garnet is related to the calcium enrichment of original rock components.
Figure 4-8 Composition Diagram of Garnet Profile
(According to Zona Lake in winter)
The right picture is an enlarged view of the lower part of the left picture.
The composition of 26 amphibole probe analysis points (including retest points) has the following characteristics: ① The light green component in the core of metamorphic amphibole is actinolite, and the blue-green component in the edge is magnesium amphibole or tremulous amphibole, which reflects a progressive metamorphic process. ② The residual amphibole core of metamorphic volcanic rocks (or intrusions) is yellow, and its composition belongs to actinolite. The transition zone is light green, and the blue-green amphibole at the rear edge belongs to magnesium amphibole. This shows that amphibole crystal chips of volcanic rocks first undergo metamorphic transformation at low temperature, and then the metamorphic temperature continues to rise, and the composition of amphibole evolves from actinolite to ordinary amphibole. The amphibole residual crystal core of intrusive body and the blue-green amphibole in transition zone belong to magnesium common amphibole, but the content of Al Ⅳ in transition zone is 0.69%, which is higher than that in core (0.24%), and the light green part at the edge is actinolite, which may indicate that these intrusive bodies invaded at the peak of metamorphism and regressed after further shear deformation. ③ The amphibole in the core of the structural lens of Xiaomiao Group is composed of magnesium amphibole-actinolite, and the edge is iron amphibole, indicating that the growth from the core to the edge is a heating process.
The plagioclase composition of plagioclase metamorphic intrusive body belongs to albite (Xab = 0.98 0.0 1), the plagioclase phenocrysts of garnet plagioclase adamellite schist also belong to albite (XAb=0.99), and the metamorphic volcanic rocks (close to gneiss) and epidote amphiboles are Xan = 0.23 ~ 0.65438. Matrix plagioclase XAn=0.22, which is generated from garnet albite amphibole in the late Xiaomiao Group and belongs to plagioclase. Early plagioclase phenocryst xan = 0.0 1 belongs to albite species.
In mica quartz schist, the muscovite in mica quartz schist belongs to normal muscovite with XK = 0.98, and the biotite with Si = 2.83 and XMg=0.44 belongs to normal biotite species. The muscovite XK = 0.93 of garnet plagioclase albite schist in Xiaomiao Group structural block is also a normal muscovite. The biotite with Si = 0.279 and XMg=0.28 is iron-rich biotite. In garnet albite amphibolite, muscovite wrapped in albite and muscovite related to deformation in metamorphic rocks or intrusions are also normal muscovites.
Chlorite is mostly dense chlorite (xmg = 0.56 ~ 0.68, si = 5.6 0. 1), and a few are vermiculite (XMg=0.46, si = 5.4).
(2) metamorphic conditions and p-T trajectories
According to the transformation reaction between actinolite and aluminum-based amphibole, the peak metamorphic condition of structural blocks of Wanbaogou Group-Nachitai Group and Xiaomiao Group should be T≥500℃. The garnet biotite mineral of garnet biotite schist of Wanbaogou Group-Nachitai Group is calculated to be 540℃ by thermometer, and the ordinary amphibole-plagioclase thermometer (Plyusinina, 1982) of green curtain amphibole is p = 0.85 GPA, t = 520℃.
The p-T trajectory of greisen schist and metamorphic rock fragments reconstructed by mineral thermometer combined with the variation of garnet composition zonal profile is shown in Figure 4-9.
(3) metamorphic age
The heating ages of amphibole in metamorphic volcanic rocks and mylonite in greisen schist are measured at 39Ar-40 Ar stage, which are 426 Ma and 408ma, 438+0 ∶ 250,000), and the age spectrum of muscovite is shown in Figure 4-65,438+00. Because the metamorphic temperature of Wanbaogou Group-Nachitai Group is 500 ~ 550℃, which is close to the sealing temperature of amphibole to Ar isotope, the related tectonic plane should have been formed before 426 Ma, and the metamorphic time related to it should be equal to or longer than 426 Ma. The zircon U-Pb age of the metamorphic intrusive body is 428 Ma (1∶ 250,000 Donggaicuona Lake), so it can be determined that its metamorphism should have occurred before 426 ~ 428Ma, and it belongs to the product of late Caledonian structural metamorphism.
(4) Analysis of metamorphic stages
In the main distribution range of Wanbaogou Group and Nachitai Group, a few structural blocks are involved in Mesoproterozoic Xiaomiao Group due to structural shear, and there are certain differences in metamorphism between them. Wanbaogou Group and Nachitai Group experienced Caledonian metamorphism, while the structural blocks of Xiaomiao Group experienced Jinning and Caledonian metamorphism.
Wanbaogou Group-Nachitai Group
The main body experienced dynamic thermal metamorphism in the East Caledonian period and ductile-brittle dynamic metamorphism superimposed in the later period. The records of regional dynamic thermal metamorphism are kept in the weak strain domain. The columnar amphiboles in massive neutral volcanic rocks are arranged radially, and the superposition of dynamic metamorphism makes the amphiboles of magmatic origin turn into chlorite. On the profile of Guli area, metamorphism is weakly zonal, with blue-green amphibole+actinolite+albite+chlorite in the south, which belongs to greenschist facies, and garnet+blue-green amphibole+plagioclase/albite in the north, which belongs to low-and medium-pressure amphibolite facies (Figure 4- 1 1), and the boundary of facies is gradually changed, so it is difficult to define.
Figure 4-9 p-T Trajectory of Caledonian Metamorphism of Wanbaogou Group-Nachitai Group in Guli Area
(According to1:250,000 Donggaicuona Lake)
GL- blue schist facies; GS- greenschist facies; Zeo-zeolite phase; EA- epidote amphibolite facies; Am- amphibolite facies; Gn- granulite facies; Eclogite facies
Fig. 4- 10 heating age map of muscovite stage in pre-nagel ultraschist in the middle section of Dongkun ophiolite melange belt.
(According to1:250,000 Donggaicuona Lake)
Fig. 4-1/Low amphibolite facies mineral map in northern nagel ultraschist before.
(According to1:250,000 Donggaicuona Lake)
Xiaomiao rock group structural block
The characteristic mineral assemblage of Jinning metamorphism is garnet+actinolite+blue-green amphibole+muscovite+epidote+syenite, garnet+muscovite+biotite+syenite, which constitutes early schistosity and is enclosed in Caledonian metamorphic albite phenocrysts. The characteristic mineral assemblage of Caledonian metamorphism is plagioclase+garnet+ordinary amphibole+epidote+albite+syenite, and plagioclase is fine in particle size and mostly sandwiched between ordinary amphibole; Albite generally forms porphyritic crystals, and its main body is further thickened on the basis of previous metamorphism, but all belong to low amphibolite facies metamorphism.
Metamorphic rocks of Nachitai Group in the north of Bukadaban Peak.
It is NW-trending and distributed in Kuru Petrica River, north bank of Haxia River and middle and upper reaches of Elgongsaiyitu River, belonging to Halabaigou Formation of Nachitai Group. Thermal contact metamorphism and dynamic metamorphism are superimposed in different degrees from west to east. Metamorphic rock assemblage is mainly a set of shallow metamorphic terrigenous flysch sedimentary clastic rocks and argillaceous rocks with a small amount of shallow metamorphic basic volcanic rocks. The original sedimentary composition, sedimentary structure and structure of metamorphic rocks, such as sandy texture, argillaceous texture, cementation method and various bedding plane structures, are well preserved or intact. The argillaceous rocks formed slate and phyllite, retaining some original sedimentary components. However, the dry power of medium-fine clastic rocks is relatively high, only metamorphic matrix and cement recrystallize, and a few form phyllite, while the original rock characteristics are well preserved.
The metamorphism of the above-mentioned rocks is mainly the recrystallization of miscellaneous bases and cements in clastic rocks and clay minerals in argillaceous rocks, while the degeneration of pre-existing minerals is the main way in volcanic rocks, with a low degree of metamorphism. The main metamorphic mineral assemblages are as follows:
Sandstone: ser+CHL+qz bit, ser+CHL+cal bit, ser+qz+cal bit, ser+CHL+qz+cal bit, ser+qz+ab+cal bit.
Mudstone: ser+CHL cal, ser+CHL qz, ser+CHL+cal+qz bit.
Basalt: EP+QZ+CAL+URL, EP+CHL+CAL.
The above mineral assemblage is represented by SER+CHL+QZ, which is common in argillaceous rocks and is characterized by the first appearance of biotite and albite. Metamorphic facies belts can be divided into biotite belts and slate phyllite low greenschist facies. Albite is a low-temperature mineral, the lower limit of its stable equilibrium temperature is 200 ~ 300℃, while the mineral combination of bit+CHL+QZ is generally less than 0.4~0.5 GPa, and its metamorphic phase system should be low-medium pressure phase system. According to the diagenetic age of this set of strata, it is inferred that the metamorphic period should be in the middle and late Caledonian.
Comprehensive analysis of the rock characteristics of sandstone and phyllite shows that the rock group has undergone at least two superimposed transformations of metamorphism and deformation. Metamorphic sandstone, slate, phyllite and other low greenschist facies metamorphic rocks were formed by early low-temperature dynamic regional metamorphism. The mineral assemblage is represented by SER+CHL+QZ, characterized by the first appearance of biotite and albite. Metamorphic facies belts can be divided into biotite belts and slate phyllite low greenschist facies. The deformation traces are S 1 foliation and thousand foliation; The metamorphic period is the middle and late Caledonian. The second stage is the superposition period of heterogeneous dynamic metamorphism, in which some greenschist facies detritus or argillaceous mylonite and phyllite are locally formed, and the deformation features are S2 mylonite foliation and local ductile shear zone, generally s 1∑S2.
2. Metamorphism of Kuhai Group in accretionary wedge Complex Belt in the south of eastern Kunming.
Mainly distributed in the area of Deshilonggou-Delongwagou and the southwest of Zhiyi Primary School. As a whole, it is a huge lens, which is distributed in Jinmen area. It is mainly composed of medium-deep metamorphic Precambrian metamorphic rocks, and it intrudes into Paleozoic quartz diorite-monzodiorite-granodiorite. Metamorphic rocks are generally migmatized, reaching low amphibolite facies, and flow folds are developed, including amphibolite-felsic section and greisen schist section. The main lithology of the former is amphibolite, amphibolite gneiss, slightly inclined migmatized amphibolite, migmatized biotite (amphibole) adamellite granulite, garnet biotite amphibolite adamellite dark gneiss, garnet biotite adamellite granulite, biotite amphibole gneiss, garnet biotite amphibole and migmatized biotite amphibole. There are many dendritic migmatized or metamorphic biotite monzogranite, metamorphic amphibole diorite and occasionally undeformed amphibole. Obviously, this is a set of volcanic-sedimentary rock series that has reached amphibolite facies metamorphism. Feldspar usually contains chalcopyrite. The main lithology of Yunying schist is cross biotite quartz schist, amphibole schist, plagioclase amphibole, light gray chronological sandstone, metamorphic glutenite, gray-black banded marble, pyrite-bearing chronological marble and tremolite marble, which are usually deposited in terrigenous clastic carbonate and metamorphic amphibole facies, and migmatization is not developed.
(1) characteristic metamorphic minerals
There are garnet, amphibole and plagioclase. Among them, garnet XAlm=0.539, XGrs=0.333 in amphibolite plagioclase dark gneiss, obviously dominated by almandine and calcium almandine molecules. The amphibole in gneiss of the same kind, Si = 6. 17 1, Mg/(Mg+Fe) = 0. 164, belongs to Fe-Ca-Mg amphibole. It was cast in the granulite facies area on the Al ⅳ-Al ⅵ change map, which is obviously inconsistent with the actual mineral assemblage related to amphibole. Plagioclase is a Labrador rock with XAn=0.625 and XAb=0.36, which is rich in calcium, which may be related to the overall calcium enrichment of the rock.
(2) metamorphic conditions
The formation temperature of garnet amphibole plagioclase is about 600℃ calculated by Pierre Chuk's amphibole-plagioclase thermometer. It is estimated by Prisnina (1982) ∑ ALHB-CAPL thermometer-manometer that it is t = 556℃ and p = 0.7 GPA. On the Si-Al Ⅳ diagram, the amphibole component point is located in the low-pressure genetic area, which may be related to the mineral assemblage without epidote, which is not suitable for the thermometer. Combined with mineral combinations of various rock types, especially in the Yunying schist of Kuhai Group, staurolite and other characteristic metamorphic minerals are produced, and their metamorphic conditions are roughly determined to be 550 ~ 600℃, with P ≤ 0.7 GPA. This result shows that the metamorphism of Kuhai rock group in this area belongs to middle-low pressure amphibolite facies.
(3) metamorphic period and metamorphic age
The metamorphic mineral assemblage of amphibolite-felsic schist and mica schist in Kuhai Group is relatively simple, and there is no superimposed mineral growth phenomenon, which shows the thoroughness of metamorphic transformation, and it is impossible to judge whether there are multi-stage metamorphic events from the perspective of petrography. The diorite gneiss is superimposed on the early regional foliation, indicating that there may be metamorphism earlier than diorite intrusion, and its metamorphism time should be more than 446 Ma, which may be equivalent to Jinning metamorphic age. The amphibole plateau age of slightly metamorphic gneiss amphibole 39Ar-40 Ar is 488 Ma (Figure 4- 12), while the zircon U-Pb harmonic age is 446 Ma, indicating that there is excess Ar in amphibole, which cannot be used as the age basis for metamorphism or deformation (Chen Nengsong et al., 2000). According to the regional comparison, the second metamorphic age of the rock mass and Kuhai Group can be defined as Caledonian, which is roughly equivalent to the metamorphic age of Nachitai Group, ranging from 426 to 428 Ma. The combination of stable plagioclase and common amphibole in basic metamorphic rocks belongs to amphibole facies metamorphism. However, it lacks indicator minerals of metamorphic facies series. According to migmatization and well-developed magmatic intrusions, it is inferred that Caledonian regional metamorphism belongs to medium-low pressure type.
Fig. 4- 12 Ar-Ar heating age spectrum of amphibole.
(According to1:250,000 Donggaicuona Lake)
The typical metamorphic minerals of Kuhai Group in the north of Lake Arak are sillimanite, garnet and biotite. Sillimanite is long columnar, sericitized at the edge, distributed in layers and coexisted with potash feldspar. Granite is a fine, irregular, granular, pink and colorless isotropic body, which coexists with potash feldspar and biotite. In the relationship diagram between garnet composition and metamorphic zone, it is mostly concentrated in sillimanite zone (Figure 4- 13). Biotite is fine scaly, dark reddish brown, parallel and alternating, and chloritization degeneration; On the composition and metamorphic belt map of biotite, it is distributed at the junction of amphibolite facies and granulite facies (Figure 4- 14). The above-mentioned metamorphic mineral composition and assemblage characteristics reflect that the main stage metamorphism of Kuhaiyan Group reached high amphibolite facies, and then according to the fact that potash feldspar+garnet and potash feldspar+sillimanite are medium-low pressure metamorphic minerals, it is inferred that the main stage metamorphism of Kuhaiyan Group can be classified into medium-low pressure metamorphic facies series.
Fig. 4- 13 schematic diagram of the relationship between garnet composition and metamorphic belt
(According to1:250,000 Arak Lake)
Fig. 4- 14 Relationship between biotite composition and metamorphic belt
(According to1:250,000 Arak Lake)
According to the metamorphic mineral assemblage (Table 4-5), the Kuhai rock group has undergone at least two metamorphisms. Early regional dynamic heat flow metamorphism caused the Kuhai rock group to undergo high amphibolite facies metamorphism, resulting in metamorphic mineral assemblages of potash feldspar+plagioclase+biotite+garnet+sillimanite and amphibole+plagioclase+biotite. The late low-temperature dynamic metamorphism produced the mineral combination of epidote+sericite+chlorite.
Table 4-5 * * Representative Metamorphic Mineral Assemblies of Kuhai Group
The metamorphic minerals biotite and garnet in the Kuhai Group in Lake Arak area were mapped by the Pilchuk biotite-garnet thermometer, and two temperature ranges of 580 ~ 660℃ and 430 ~ 470℃ were obtained. According to Pb-Pb age of zircon in garnet-bearing biotite plagioclase gneiss, it is 70617 ma (1:250,000). The combination of potash feldspar and garnet in rocks represents the medium-pressure metamorphic environment, and it is speculated that the paleo-sea rock group was in the collision orogenic structure environment during Jinning period. The metamorphic temperature environment of Caledonian Kuhai Group should be between 430 and 470℃. In addition to the above, there are signs of metamorphism in the Qiannanhua geological body in the north of East Kunlun, and there are also signs of metamorphism in Subhas and Kudinan of West Kunlun.