5.1 Magmatic origin
In the Harker diagram (Fig. 17), from granodiorite porphyry → biotite monzogranite, as SiO2 increases, Al2O3, FeO, CaO, MgO, MnO, Na2O, P2O5, TiO2 decrease, and K2O increases, which may be due to Due to the separation and crystallization of plagioclase, titanium-iron oxides, mafic minerals and apatite from the melt, the entire magma evolution stage shows an obvious crystallization differentiation process.
According to the SiO2-Ce and SiO2-Zr diagrams of the petrogenetic series (Fig. 18), the granodiorite porphyry, the biotite monzogranite and the diorite enclaves in the biotite monzogranite are all cast in the I-type granite.
The mineral composition of granodiorite porphyry is similar to that of biotite monzogranite, and it is mainly composed of potassium feldspar, plagioclase, quartz and amphibole. Compared with biotite monzogranite, granodiorite porphyry contains less quartz, more plagioclase, dark minerals are common amphibole, and biotite is rare or absent.In terms of major element content, from granodiorite porphyry → biotite monzogranite, the content of SiO2, total alkali content (Na2O + K2O), K2O/Na2O, and alkalinity ratio (AR) gradually increased; The index SI shows a decreasing trend; the differentiation index DI increases linearly, reflecting the increasing degree of differentiation of magma.The granodiorite porphyry and biotite monzogranite have very consistent distribution characteristics of trace element spider web diagrams and rare earth element distribution diagrams.Wang Liang et al. (2011) proposed that the magma will cause changes in the composition of the magma system during the process of its differentiation and evolution. The behavior of different trace elements in the process of magma evolution has its own characteristics. The study of trace elements can be used to judge magma. Differential evolution degree. Rb/Sr is the most obvious indicator of magma evolution.Some trace elements in the magmatic rock mass in the working area show (Table 4), from granodiorite porphyry → biotite monzogranite, the Rb/Sr ratio is on the rise, the Nb/Ta ratio, Zr/Hf ratio, K/Rb ratio And the Ba/Rb ratio showed a downward trend.The ratio of trace elements shows that there is a differential evolution relationship between granodiorite porphyry and biotite monzogranite.Fu Qiang et al. (2011) and Sang Xuezhen (2013) pointed out that in the process of homologous magma differentiation and evolution of rare earth elements, with the separation and crystallization of plagioclase, potassium feldspar and other minerals, the negative europium anomaly of residual magma will tend to obvious.The average δEu of granodiorite porphyry in this study area is 0.66, and the average value of δEu of biotite monzogranite is 0.36.According to the change of δEu, it can be clearly seen that the homologous magma differentiation and evolution relationship from granodiorite porphyry to biotite monzogranite.The zircon U-Pb age of granodiorite porphyry is 220.7 ± 1.3 Ma; the zircon U-Pb age of medium-coarse-grained biotite monzogranite is 220.4 ± 1.6 Ma; The fine-grained biotite monzogranite zircon U-Pb age is 220.5 ± 1.6 Ma.It is inferred that the granodiorite porphyry and the biotite monzogranite were formed at the same time, and the formation sequence of the rocks in the magmatic body in the working area should be granodiorite porphyry→biotite monzogranite.
At present, there are various genetic classifications of enclaves (Mass et al., 1997; Xiao Qinghui, 2002; Zhou Jincheng et al., 1992), which can be roughly divided into the following three categories: ① Surrounding rock captivity (Mass et al., 1997;) .The surrounding rock of the Volcano granite in the study area is mainly metamorphic quartz sandstone of the Tumugou Formation, and the dark diorite enclaves in the biotite monzonitic granite are mostly round-shaped, which are quite different from metamorphic quartz sandstone in composition, and are basically excluded. Origin of surrounding rock captive bodies.②The basal residue (Xiao Qinghui, 2002). The dark diorite enclaves in the study area have a magma structure, and there are no aluminum-rich minerals such as spinel, andalusite, muscovite, and no schistose structures, which can basically rule out the possibility of basement remnants.③Magma immiscible inclusions (Zhou Jincheng et al., 1992). It refers to the inclusion of granite magma which is separated into two conjugated immiscible phases due to thermodynamic instability under higher temperature conditions, and the partial neutral phase crystallizes in the partial acid phase.
The surrounding rocks of the biotite monzogranite in the study area are mainly metamorphic quartz sandstone and slate of the Tumugou Formation, with characteristic metamorphic structure and structure; while the dark diorite enclaves in the biotite monzogranite have a magmatic structure, there is no aluminum-rich minerals such as spinel, andalusite, muscovite, and no schistose structure, which can basically rule out the possibility of surrounding rock capture bodies and basement remnants.After ruling out the possibility that the diorite enclaves in the biotite monzonitic granite are the captive bodies of the surrounding rock and the remnants of the basement, what is the origin of the diorite enclaves?
The boundary between the dark diorite inclusions and the host biotite monzogranite is obvious.However, the diorite inclusions and the host biotite monzogranites have similar mineral compositions, the main components are potassium feldspar, plagioclase, quartz, biotite, amphibole and so on. The similarity of the mineral composition shows the homology of the two.The difference is that the dark diorite inclusions are darker in color and finer in particle size, with more dark minerals than the host biotite monzogranite, and slightly less potassium feldspar and quartz.The thin columnar apatite seen in the dark diorite inclusions may be the product of rapid condensation and crystallization of the inclusions.Occasionally, giant porphyritic potassium feldspar is found in the dark diorite enclaves, which may be the product of acid magmatism after the separation of the diorite enclaves from the host biotite monzogranite.The chemical composition of diorite inclusions is compared with that of biotite monzonitic granite (Table 4.1), the oxide content of iron, magnesium and calcium is relatively high, and the oxide content of potassium is relatively low.The ratios of Mg/(Mg + Fe) of the dioritic inclusions and the host are 0.16 ~ 0.20 and 0.07 ~ 0.22, respectively, with an average value of 0.18 and 0.15, respectively; the ratios of Na(Na + CaO) are 0.54 ~ 0.86 and 0.54 0.49 ~ 0.57, the ratio is similar, which proves that iron, magnesium, sodium and calcium were not separated during the formation process. The two are similar in origin.
The distribution characteristics of trace elements in the dark diorite inclusions and the host are basically the same, in which the compatible element Rb is close, and the content of the incompatible elements Sc, Co, V, Ni, Y in the diorite inclusions is higher than that in the biotite monzogranite. The high level may be caused by the formation of dark inclusions, which is consistent with the characteristics of immiscible inclusions proposed by Watson (1976) and Zhou Xinmin (1992).The curves of the two are similar (Fig. 22), reflecting their evolutionary affinities. The distribution diagram of rare earth elements between the dark diorite inclusions and the host (Fig. 8) and the trace element cobweb diagram (Fig. 22) are very consistent, indicating that the diorite inclusions are homologous inclusions of the host rock, biotite monzonitic granite.From the characteristic values of rare earth elements in the magmatic rock mass in the working area (Table 5), it can be seen that the total amount of rare earths in dioritic inclusions is higher. Compared with the host body, the content of light rare earths decreases, the content of heavy rare earths increases, and the ratio of LREE/HREE increases.The average δEu of the diorite enclaves is 0.38, and the average δEu of the biotite monzogranite is 0.36, both of which have obvious negative europium anomalies. It is related to the precipitation of plagioclase when the magma is immiscible, and the giant phenocryst potassium feldspar formed after the late metasomatism can be seen under the microscope of diorite inclusions, indicating that the inclusions are associated with acid magmatism.Through this zircon U-Pb age study, the dark dioritic inclusion zircon U-Pb age is 219.8 ± 1.4 Ma, which is consistent with the monzogranite zircon U-Pb age (220.4 ± 1.6 Ma, 220.5 ± 1.6 Ma )similar.
Combined with petrographic and mineralogical characteristics, rock geochemical analysis results and zircon U-Pb chronology studies, the diorite enclaves in granodiorite porphyry, biotite monzogranite and biotite monzogranite should be A set of rock assemblages formed by the evolution of the same magmatic melting event, that is, the three are the products of homologous magma evolution.Among them, biotite monzogranite and granodiorite porphyry are the products of homologous magma crystallization differentiation, and the magma evolution sequence is: granodiorite porphyry→biotite monzogranite.The formation of diorite enclaves in biotite monzonitic granites is the result of self-evolution of the granitic magma when it undergoes a fusion-immiscibility process.Its origin is equivalent to the homologous early liquid immiscible inclusions classified by Ma Changqian (1992) and Xu Xisheng (1993).
5.2 Analysis of tectonic environment of magmatic rock mass
The geochemical characteristics of magmatic rocks are quite different due to the different formation environments of magmatic rocks. The study of the geochemical characteristics of magmatic rocks will help to clarify the tectonic environment produced by magmatic rocks.The material composition of magmatic rocks is subject to the composition of the source area, and the geochemical characteristics of trace elements of magmatic rocks formed in different tectonic environments are also significantly different.
In the R1-R2 covariation diagram (Fig. 19), the granodiorite porphyry mainly falls in the area before plate collision; while the biotite monzogranite mainly falls in the late orogenic area, which may indicate the formation of granodiorite porphyry The age is earlier, the biotite monzogranite is slightly later.
The analysis data was projected using the Hf-Rb/10-Ta×3 and Hf-Rb/30-Ta×3 triangular diagrams (Fig. 20), and the Hf-Rb/10-Ta×3 diagram (Fig. 20A) showed The granodiorite porphyry and biotite monzogranite are cast in the volcanic arc granite range. Near arc granite and volcanic arc granite. On the whole, the biotite monzonite granite tends to fall into the intraplate granite.
Using the classic Pearce et al. (1984) granite tectonic environment discrimination diagram for mapping, on the Ta-Yb map, most samples fall in the volcanic arc granite (VAG) area, and some fall in the volcanic arc granite (VAG) and intraplate Near the boundary of granite (WPG) (Fig. 21A).On the Rb-Yb + Nb map, the sample falls near the boundary between volcanic arc granite (VAG) and syn-collision granite (syn-COLG) (Fig. 21B).
In the Hf-Rb/30-Ta×3 (Fig. 20B) and Ta-Yb (Fig. 21B) diagrams, part of the biotite monzogranite falls within the intraplate granite range, probably due to the contrast between the biotite monzogranite and the granite Long porphyry was formed in a relatively stable environment of tectonic activity, reflecting the evolution process from volcanic arc granite to intraplate granite.
Combined with the normalized trace element spider web map of the original mantle (Fig. 22), granodiorite porphyry, biotite monzogranite and diorite enclaves are relatively depleted in high field strength elements (HFSE) and enriched in large ion lithophile elements ( LILE), similar to volcanic arc granites. According to the above, it can be seen that the product of magmatism under the tectonic background of the granite volcanic arc in the Volcano area.
Combined with the analysis results of trace and rare earth elements in the samples, the curve of the Yidun Island Arc Volcanic Rock is in good agreement with the middle-acid volcanic rocks of the Tumugou Formation (data not yet published) and the upper crust curve, indicating that the Yidun Island Volcano intermediate-acid intrusion is closely related to the middle-acid intrusion. The acidic volcanic rocks in the Tumugou Formation are homologous magmas, and the source material comes from the melting of the upper crust, so the formation environment should also belong to the volcanic island arc tectonic environment.
At the same time, zircon U-Pb chronology studies also show that the diagenetic age of granodiorite porphyry, biotite monzogranite and Tumugou Formation medium-acid volcanic rocks (data not yet published) of the granite bodies in the Volcano area is 220 Ma. It is consistent with the volcanic-magmatic peak period (228 ~ 213Ma) of the Yidun Island arc considered by Hou et al. (1995).
Based on the above discussion, it can be seen that the granite bodies in the Volcano area were formed in the volcanic island arc tectonic environment.
5.3 Tectonic evolution
Hou Zengqian et al. (2001) believed that the diagenetic age of the granites in the Yidun-Shaluri island arc belt arc was 237 − 206 Ma; Wang et al (2013) believed that the southern segment of the Ganzi-Litang oceanic crust began to subduct westward at 230 Ma, and the northern segment began to subduct westward at 230 Ma. Subduction started at 224 Ma; the latest U-Pb zircon dating results of the Yidun-Saluli arc granite granites range from 225 to 215 Ma (Reid et al., 2007; Weislogel et al., 2008; Peng et al., 2014 ); the zircon U-Pb age of the second member of the Tumugou Formation rhyolite is 219 Ma, the zircon U-Pb age of the granodiorite porphyry in the Samalong depression is 220 Ma, and the Yidun-Shalu age is 220 Ma. The zircon U-Pb age of the granite in the Volcano area in the middle section of the Lidao arc belt is 220 Ma. The above research results show that the Litang Ocean Basin may have subducted from east to west at the earliest in the Middle Triassic, forming the Yidun Island Arc in the west., and continued until the Late Triassic.
Based on the above research results, it is possible that the Litang Ocean Basin subducted from east to west at the earliest in the Middle Triassic, and continued until the Late Triassic. Upwelling of mantle material in the asthenosphere leads to thinning of the lithosphere and delamination (Pearce et al., 1990; Aldanmaz et al., 2000; Ilbeyli et al., 2004), resulting in partial melting of the metasomatic mantle wedge material, forming basaltic magma (Fig. 23a).With the continuous increase of basaltic magma, a basaltic magma chamber was formed at the contact interface between the lower crust and the mantle (Fig. 23b).At around 220 Ma, with the continuous increase of basaltic magma chambers, a large amount of heat energy was transferred to the upper crust, resulting in the melting of the upper crust and the formation of high-silicon, high-alkali, and aluminum-supersaturated granitic magmas (Fig. 23c) (Huppert and Sparks, 1988; Chalot-Prat, 1995; Zhu et al., 1996; Zhu Yongfeng et al., 1995), these granitic magmas are the original magmas of the granite complex in the Volcano area.Part of the granitic magma rose to the near surface after diapiring and intruded into the granodiorite porphyry in the southwest of the work area.Most of the remaining magma was further crystallized and differentiated, and the partial neutral phase was separated from the thermodynamic instability at higher temperature. The enclaves crystallized in the granitic magma rose to the near surface to form biotite monzogranite, and a large number of biotite monzogranites developed. Dark particle inclusion group (Fig. 23d).