Sanjiang Tethys developed to Triassic, and its main bodies, Jinsha River-Ailaoshan Ocean and Lancang River Ocean, closed into collision orogeny stage, and Ganzi-Litang Ocean subducted into closure stage. The products of magmatism on the boundaries of these polymer plates have both individuality and individuality. It is characterized by the formation of island arc volcanic rocks and collision granitoids with similar geochemical characteristics, but the products formed at different boundaries and stages have their own characteristics due to the different composition and thermal state of the crust and mantle.
1. Changtai-Xiangcheng island arc magmatic belt
Changtai-Xiangcheng island arc has developed well and matured in a short period of time. From morning till night, it can be divided into pre-island arc period, main island arc period and late island arc period. The trench-arc-basin system develops from east to west in space. During the pre-island arc period (), a set of rift alkaline transition basalt with high TiO _ 2 or bimodal combination of basalt and rhyolite was formed, and typical boehmite with high MgO, SiO _ 2 and extremely low TiO _ 2 was also produced in the southern part of Xiangcheng. The main island arc period (1920- 1930) can be divided into three stages. In the early and late stages of arc formation, the calc-alkaline series basalt-andesite-dacite-rhyolite assemblage dominated by andesite was formed, and the inner arc (Xi 'anshan rock belt) of the outer arc (Donganshan rock belt) of the main arc area was formed respectively. In the inter-arc rift stage, a bimodal combination of rhyolite and tholeiite was formed, which distributed in the inter-arc rift zone (rift zone) between the inner and outer arcs, and the famous Xiacun massive sulfide polymetallic deposit was produced at this stage. In the late arc, high-potassium basalt or sylvite (Figure 8-4) and rhyolite developed in the back-arc basin. The above characteristics show that the Changtai-Xiangcheng island arc has experienced a complicated history of alternating tension and compression, which is different from other volcanic arcs in Sanjiang area (Mo Xuanxue, Lu Fengxiang, etc.). , 1993). In the K2O-SiO2 _ 2 diagram (Figure 8-5), the back-arc potash is located in the potash series area, and other volcanic rocks are distributed in the middle-potassium-calcium-alkali series area, with high K2O content, which is close to the boundary of high-potassium series, suggesting that there is a certain genetic relationship between the volcanic rocks in the pre-arc period and the main arc period.
During the formation of the island arc, a small number of calc-alkali series hypabyssal intrusive rocks, mainly diorite, were formed with volcanism, including Ranmuke diorite, quartz diorite on the left slope, Mucuo plagioclase granite, Xuejiping diorite porphyrite and so on. The K2O- silica diagram (Figure 8-5) is consistent with the distribution range of island arc volcanic rocks, and it was formed earlier (about 237 ~ 220 years). The main granitoids in this belt were formed after plate collision, including stagger horse, Yongjie and Dongcuo bedrock. The main lithology is granodiorite and adamellite, with a small amount of potash granite, which is in intrusive contact with late Triassic island arc volcanic rocks and sedimentary rocks, and its formation age is between 220 and 208 Ma. In the K2O-SiO2 _ 2 diagram (Figure 8-5), it is distributed in the high-potassium-calc-alkaline series area, with higher K2O than volcanic rocks and intrusive rocks in the island arc period, and its geochemical characteristics have the characteristics of type I granite (Lu Boxi et al., 1993).
2. Jiangda Weixi Lvchun arc magmatic belt
Judging from the development of volcanic rocks, the collision time of Jinsha River-Ailaoshan Ocean Plate is different in different sections. In the northern section (Jiangda-Weixi section), the collision may occur from the end of late Permian to early Triassic, because late collision and lagging volcanic rocks appeared in early Triassic.
There is no single rhyolite formation or rock segment in the Jiangda-Chesuo section, and the volcanic rocks from the Lower Triassic to the Middle Triassic are all basic-neutral-acidic calc-alkaline series. The Lower Triassic (Pushuiqiao Formation and Serong Formation) contains basalt (a small amount), andesite, andesite volcanic breccia and tuff. Volcanic rocks are 300 meters thick, accounting for 32% of the formation, mainly pyroclastic rocks (285 meters), and sedimentary rocks are continental and coastal facies. Middle Triassic (Wallasi Formation) interbedded with andesite, andesite volcanic breccia, tuff and slate, with volcanic rock thickness of 686m, accounting for 25% of the formation, and more pyroclastic rocks (583m). Upper Triassic (Jiangda Formation) is the main part of this arc volcanic rock, which is a combination of calc-alkaline series basalt-andesite-dacite-dacite rhyolite and rhyolite, and the content of pyroclastic rocks is also high, including intermediate-acid tuff, breccia tuff and molten volcanic breccia.
Figure 8-4 TAS Diagram of Triassic Volcanic Rocks in Sanjiang
(According to Lebas 1986)
I—Irvine( 197 1) the dividing line between alkaline series (a) and subalkaline series (s); B- basalt; O1-basaltic andesite; O2- andesite; O3— dacite; R- rhyolite; S1-Hawaiian rock (Na) and potash trachyte basalt (k); S2-olivine trachyandesite (Na) and potassium salt (K); S3-plagioclase trachyte (Na) and andesite (K); T-trachyte (q < 20%) and trachyte dacite (q > 20%). 1- Changtai-Xiangcheng arc volcanic belt; 2- Jiangda-Weixi Luchun arc volcanic belt; 3- Heteropoly-Yanjing-Jinghong Arc Volcanic Belt
In several sections of Ding Wei-Weixi, rhyolite and pyroclastic rocks with high silicon (SiO _ 2) and high potassium (K2O) are produced in the Lower Triassic (Marathon Formation) and the Upper Triassic (Pantiange Formation), with contents of 70.75% ~ 78.75% and 2.64% ~ 5.32% respectively. Iron peridotite altered rhyolite occurs in Longqiao, Haitongjiao. In the K2O- silica diagram (Figure 8-5), they are distributed in high-potassium calcium-alkali series areas.
Triassic volcanic rocks in Taichung-Lu Chun section are mainly exposed in Lu Chun area, only in Upper Triassic, and the lower part is a set of co-collision rhyolite with high SiO _ 2 (73.39%) and K2O(5.20%), which is distributed in Luchun Gaochanzhai area. The upper part is andesite and intermediate-acid pyroclastic rock of sylvite series (Figure 8-5).
Granitoids in Jiangda-Weixi-Lvchun island arc magmatic belt are younger than 235Ma, ranging from 235- 194Ma, and most of them are younger than 2 17Ma (according to Lu Boxi et al. , 1993), that is, they were formed in the late Triassic. In addition, intrusion into the Late Triassic strata can be seen in some places, such as Jiaduoling rock mass, Ludian rock mass and Ludong rock mass. Based on this, it can be considered that the granite intrusive rocks in this area were mainly formed after the late Triassic lag arc volcanic rocks, belonging to the late collision granite. There are various types of rocks in Jiangda-Weixi section, including diorite porphyrite, diorite porphyrite, quartz diorite, diorite, granodiorite, adamellite, granite, granite porphyry and a small amount of amphibole syenite. Taichung-Lu Chun section is simple in lithology, and all are adamellite (according to Lu Boxi 1993). K2O of granite shows two characteristics (Figure 8-5). Some of them are distributed in the middle-potassium-calcium-alkali series areas with volcanic rocks in this zone, and there may be some genetic relationship between them. Most of them are distributed in high-potassium calcium-alkali series areas.
3. Heteropoly-Yanjing-Jinghong arc magmatic belt
After the collision of plates in this belt in the early Triassic, the co-collision rhyolite was formed from north to south in the middle Triassic. Late Late Late Triassic collision or lagging volcanic rocks developed unevenly.
Figure 8-5 K2O- silica diagram of Triassic magmatic rocks in Sanjiang area
(According to Le Maitre,1989; Lofgren, 198 1)
Lk-ca- low potassium calcium base series; Calcium-calcium-alkali series; HK-CA- high potassium calcium alkali series; Sho-potassium salt series; 1, 4- volcanic rocks and granitoids in Changtai-Xiangcheng belt; 2.5— Volcanic rocks and granitoids in the west Luchun belt of Jiangdawei; 3.6— Volcanic rocks and granite in Jinghong area of Zaduo salt well.
Granite is the representative composition (according to Lu Boxi1993); Volcanic rocks are general.
In the northern section of Zhuka-Yanjing section, dacite, rhyolite and high silicon rhyolite are widely exposed, and some dacite contains fayalite. Pyroclastic rocks account for 75%, and there are many kinds, such as tuff and tuff. The total thickness of volcanic rocks is 8268m, and most rhyolite rocks are characterized by high SiO _ 2 (69% ~ 73%) and K2O (greater than 3.0%, with an average of 4.6%), which are distributed in high-potassium calcium-alkali series areas (Figure 8-5). There is no delayed volcanic rock production above.
Arc volcanic rocks are widely developed in Yunxian-Jinghong section of the southern section, which are distributed from north to south along the southern Lancang River. After the subduction of Permian synchronous arc volcanic rocks, they are syncollision rhyolite in Middle Triassic and late collision or lagging arc volcanic rocks in Upper Triassic. One of the characteristics of this zone is that Triassic volcanic rocks have some different geochemical characteristics in the northern section (north of Jinggu Minle) and the central and southern section. The rock types of collision rhyolite in Middle Triassic are rhyolite, tuff and ignimbrite. The northern section is high potassium calcium alkali series, and the central and southern section is medium potassium calcium alkali series. The Upper Triassic (Xiaodingxi Formation and Manghuihe Formation) is a lagging arc volcanic rock, and the northern part is a high-potassium calc-alkali series-a series of potassium basalts (Figure 8-5). The rock assemblage is potassic trachyte-high-potassium basalt-potassic basalt-andesite-high-potassium rhyolite. The enrichment degree of potassium is higher than that of arc volcanic rocks in Changtai-Xiangcheng belt and Jiangda-Weixi belt in the south-central part (Figure 8-5), and it belongs to low-medium potassium calc-alkali series. The rock assemblage is timely tholeiite-basaltic andesite-dacite-dacite, and the potassium content is lower than those in the above belts.
The granite in the western belt is dominated by bedrock, and the rock types are mainly granodiorite and adamellite. Except for the huge Lincang batholith, the other age values are all between 23 1 ~ 194 Ma, and most of them are between 230 ~ 2 17 Ma, that is, the main body was formed in the early and middle period of Late Triassic, and granite intruded into the late Triassic volcanic rocks of Chayayouxi and Lincang. Therefore, the granitoids in this area belong to post-collision syncollision granites. In Figure 8-5, they are all distributed in high-potassium calcium-alkali series areas, and their geochemical characteristics are S-type granites (according to Lu Boxi, 1993).
(2) Main geochemical characteristics and source analysis.
(1) The characteristics of trace elements and rare earth elements of Triassic island arc volcanic rocks in Sanjiang area are basically the same, similar to typical island arc volcanic rocks, but different from oceanic ridges and intraplate environments. The content of TiO _ 2 is less than 2.0%, which belongs to low titanium type (Table 8- 1). This is because the magma source area of arc volcanic rocks is shallow, which generally does not reach the depth of molten rutile rich in TiO _ 2. Pearce trace element distribution pattern of volcanic rocks shows obvious characteristics of enrichment of Th and stone elements Sr, K, Rb and Ba by large ions, while Nb and Ta are lost in high field strength, Zr and Hf are partially lost, and acidic volcanic rocks also show P and Ti loss valleys. The distribution pattern of rare earth elements is light rare earth weakly enriched-moderately enriched, (La/SM) n = 2 ~ 5, increasing from basic-neutral-acidic volcanic rocks (Table 8- 1). No europium anomaly or weak negative europium anomaly indicates that the separation and crystallization of calcium-rich minerals has not occurred or is very weak.
Table 8- 1 Triassic arc magmatic rocks in Sanjiang
Some data of volcanic rocks in the table are from Mo Xuanxue and Lu Fengxiang (1993). Granite data is calculated according to the data of Lu Boxi et al. (1993).
(2) Take the Yunxian-Jinghong section of the southern section of the heteropoly-Yanjing Jinghong arc volcanic belt as an example, which is a compound arc volcanic belt. Permian calc-alkaline andesite volcanic rocks are subduction synchronous arc volcanic rocks; Middle Triassic rhyolite with high silicon and high potassium is a collision volcanic rock. Upper Triassic volcanic rocks are late collision or lag arc volcanic rocks. As mentioned earlier, there are obvious differences between the Triassic volcanic rocks in the north and the south-central part. The most prominent is the volcanic rocks in the north (folk music, Wenyu, Xiaoxing, etc. ) is rich in K2O, while the south-central part (such as the highway from west to Lanzhou) is rich in sodium. At the same time, they also show a series of geochemical differences. The Triassic volcanic rocks in the northern section (T2-T3) belong to the high-K calc-alkaline series-K rhyolite series, and belong to the combination of K trachyte-K basalt-K rhyolite. Light rare earth elements are moderately enriched, (la/sm) n = 5.32 ~15.13; Pearce trace element distribution pattern shows that Nb, Ta, Ti and Cr are deficient and K, Rb, Ba and Th are enriched (Figure 8-6). W (nb)/w (y) = 0.67 ~1.17, and W (Th)/W (Yb) is greater than 3. Triassic volcanic rocks in central and southern China belong to low-potassium tholeiite-medium-potassium calc-alkali series, and the combination is timely tholeiite-basaltic andesite-andesite-dacite; The enrichment degree of light rare earth elements is low, (La/SM) n = 3.03 ~ 7.32; Pearce's trace element distribution pattern not only loses Nb, Ti and Cr, but also loses Hf, K and Rb, and the only enriched element is Th (Figure 8-6). W(Nb)/w(Y) is less than 0.67, and w (th)/w (Yb) is less than 3. Comprehensive analysis shows that the difference between them is not caused by late alteration or metasomatism, but because they belong to different magmatic evolution series and have different sources. This is intuitively shown in Figure 8-7. The Permian and Middle Triassic volcanic rocks in the north and Late Triassic volcanic rocks in the south-central part formed four evolution trend lines. The angle between the first, second and third trend lines and the w(Ba)/w(Ca) axis is small, which indicates that the evolution of these magma is mainly controlled by separation crystallization, and the mineral phase of separation crystallization is mainly plagioclase, with a certain amount of pyroxene and amphibole. It is worth noting that the potassium-rich volcanic rocks in the north are closely related to copper mineralization, while the low-potassium volcanic rocks in the south-central part have not found valuable mineralization.
Figure 8-6 Pearce Trace Element Distribution Model of Late Triassic Island Arc Volcanic Rocks in Yunxian-Jinghong Section
SL-Silan highway; Mc-busy little star; Wy- wenyu
(3) Triassic granitoids also show similar characteristics to volcanic rocks in some aspects. Granite, like rhyolite, also belongs to low titanium type, with TiO 2 < 0.6%; The distribution pattern of rare earth is also light rare earth medium enrichment type, (La/SM) n = 3 ~ 4.5 (Table 8-1); In the K2O- silica diagram (Figure 4-39), most granites are distributed in high-potassium calc-alkali series areas, just like rhyolite of the same collision type.
There are some similarities and differences between granites in various rock belts (Table 8-2), because they are mainly formed in collision orogenic environment, but their source areas and magma modes are different. It can be seen from Table 8-2 that the total alkali content of granitoids in each zone is similar, ranging from 3.5% to 7.5%, and w(K2O)/w(Na2O) is greater than 1. The granite features of Changtai-Xiangcheng belt in the northern section of Sanjiang River are similar to those of Jiangda-Deqin belt, while those of Weixi-Lvchun belt in the southern section are similar to those of Dongdashan-Lincang belt. The geochemical types of the first two are mainly aluminum-like, with the characteristics of I-type granite; In addition to high-potassium calc-alkali series, there are a few calc-alkali series and potash basalt series (Figure 8-5). Rock types include granite and diorite; The main formation age is less than 218ma; The initial value of (87Sr/86Sr)i is less than 0.7 12. The geochemical types of the latter two are mainly peraluminum, with the characteristics of S-type granite, and only high-potassium calcium-alkali series (Figure 8-5); Rock types are all granite without diorite; The age value of main strata is greater than 218ma; The initial value of (87Sr/86Sr)i is greater than 0.7 12.
Table 8-2 Main Characteristics of Triassic Granites in Sanjiang Area
(4) The source and evolution of magma in subduction zone of convergent plate are much more complicated than other environments, so it is difficult to simulate quantitatively, because this is an open magma system (Deng, 1989), and the magma source areas are often diversified, and subducted oceanic crust, overlying wedge mantle and overlying continental crust may participate in different proportions. The main reason for the diversity of arc volcanic rocks in Sanjiang area may be the diversity of provenance components. Arc volcanic rocks are mainly neutral volcanic rocks, with a large proportion of acidic volcanic rocks and less basic volcanic rocks. According to the material composition of crust and mantle and the limitation of experimental petrology, the magma source should be mainly crust source or crust-mantle mixed source. The mixed source of crust and mantle refers to the direct participation of mantle-derived magma, but the magma formed by melting after the mantle-derived magma invaded the solidified rocks in the crust cannot be regarded as the mixed source of crust and mantle. But only the shell source code. One of the mixed source mechanisms of crust and mantle may be the underplating or intermediate intrusion of mantle-derived magma. The gabbro diorite body with similar geochemical characteristics produced by the volcanic rocks of Silan Highway is contemporaneous with the Upper Triassic, indicating that the intrusion of basic magma occurred at the same time as the formation of the upper Triassic volcanic rocks, which may lead to mixed source of crust and mantle. Rhyolites and granites distributed independently in the Middle Triassic can only be of crust origin.
In addition, the ratio of strongly incompatible elements can be used to analyze the properties of the source region, because strongly incompatible elements are less sensitive to separation and crystallization, and their element comparative value can reflect the characteristics of this ratio in the source region. In this paper, the w(K)/w(Ba)-w(K)/w(Pb) correlation diagram (Figure 8-7) is selected, and the Yingpan Ridge basalt with the largest ratio of w(K)/w(Rb) in the study area is selected to represent the end members of the oceanic crust, and the lherzolite in the ophiolite in Ailao Mountain is selected to represent the end members of the upper mantle. K and Ba of the crustal endmembers are derived from the chemical element abundance of Triassic strata in Sanjiang area (Julia, 1992). Because of the lack of rubidium in sample analysis, the crustal element abundance rubidium value of thomas lee (1976) (Zhao, Zhang Benren, 1988) is adopted. The average composition of volcanic rocks and granitoids in each zone is put into the figure, and the main points are distributed near the crustal endmembers, indicating that the Triassic island arc magmatic rocks mainly come from the crust, and the oceanic crust and upper mantle have little or no composition; Of the three basalt points in Changtai-Xiangcheng Island Arc and Yunxian-Jinghong Island Arc, two are close to the end element of oceanic crust and one is close to the end element of mantle, which may imply that basaltic magma is dominated by mantle source, and a certain amount of oceanic crust components are involved.
Fig. 8-7 Diagram of Triassic arc magmatic rocks in Sanjiang area with W (k)/W (ba)-W (k)/W (Rb)
1, 4— volcanic rocks and granitoids in Changtai town belt; 2.5— Volcanic rocks and granitoids in the west Luchun belt of Jiangdawei; 3.6— Volcanic rocks and granitoids in the heteropoly salt well-Jinghong zone; 7— Crustal composition; 8— Upper mantle composition (Ailaoshan lherzolite); 9— Ridge basalt (Yingpan). Corresponding to Table 5- 1
In addition, the REE partition model and lead isotope of granitoids can also provide some provenance information. Figure 8-8 shows the distribution pattern of rare earth elements in granitoids in each zone. The enrichment degree of light rare earth elements is similar, but the loss degree of heavy rare earth elements is different from the abnormal characteristics of Europium. From Cuojima-Dongcuo Granite-Dongdashan-Lincang Granite-Jiangda Porphyry-Zhongdian Granite, the loss of rare earth elements gradually became stronger, and the negative europium anomaly gradually weakened. Both the granite in Cuojima-Dongcuo belt and Zhongdian granite are located in the magmatic belt of Changtai-Xiangcheng island arc, and the difference of REE distribution patterns means that their diagenetic modes or source regions are different. The magma of cuojiaoma-dongcuo granite may have undergone obvious separation and crystallization of plagioclase, or there are more plagioclase in the residual phase of magma source region, resulting in obvious negative europium anomaly. The weak deficit of HREE may indicate that the source rock itself is rich in HREE, and the secondary minerals rich in HREE (such as zircon and garnet) remain less in the source area and more molten phases. Zhongdian granite slurry has hardly experienced the separation and crystallization of plagioclase, or there are few plagioclase in the residual phase in the source area, but there are many residual accessory minerals rich in HREE. The above characteristics of porphyrite in Jiangda belt and granite in Dongdashan-Lincang belt are between them. This also reflects the different provenance composition of the three granite belts, which proves that they belong to three different tectonic units.
Figure 8-9 shows that the lead isotopic composition of Lincang granite is different from that of Xuejiping porphyry copper mine in Zhongdian (galena determination). Lead isotopic composition of dispersed galena in Lincang granitoids: 206Pb/204Pb is 18.347 ~ 18.927, and 207pb/204pb is 15.458 ~ 15.894, 208pb/208pb. The lead isotopic composition of Xuejiping porphyry copper deposit is 206Pb/204Pb,17.892 ~17.913, 207 Pb/204 Pb,15.501~. The relative variation values are 0.02 1, 0.027 and 0.07 1, respectively, and the variation range is very small, and the lead isotope composition is quite uniform, indicating that the composition of its source region is also relatively uniform, which is different from Lincang granite. The composition of these two isotopes is distributed on the NHRL line (the northern hemisphere reference line composed of lead isotopes of MORB and OIB), and the long axis of the distribution area is oblique to the NHRL line, which is similar to the composition characteristics of island arc magmatic rocks undergoing subduction in the northern hemisphere, suggesting that there is a mixture of subduction oceanic crust and continental elements in its source area (white,1989; Be-bout et al., 1993). For the granite in this area, it may indicate that there is a small amount of subducted oceanic crust in its source area, or it may be the difference between delayed arc magmatism and intracontinental magmatism.
Figure 8-8 REE partition model of some Triassic granites in Sanjiang area
(Quoted from Lu Boxi, 1993)
A-REE partition model of Lincang granite belt in Dongdashan: B-REE partition model of cuojiaoma-Dongcuo granite belt: C-REE partition model of Zhongdian granite: REE partition model of porphyrite in D- Jiangda belt.
In a word, according to the analysis of the above limited data, the basaltic magma of the basic end member of the three-zone arc magmatic rocks in the study area comes from the upper mantle and has a certain amount of subducted oceanic crust. Other intermediate-acid volcanic rocks and granitoids mainly come from the earth's crust, and there may be a small amount of mantle source and/or subduction oceanic crust components. The composition of magma source areas in different rock zones is different.