Genesis and geological significance of Granitic Rocks in Eastern South-Altun Orogenic Belt

doi:10.21203/rs.3.rs-1718760/v1

Aim at the problems that the genesis and deep dynamic mechanism of Paleozoic-Mesozoic granites in the north western margin of South Altun Mountains have not been analyzed on the whole, systematic petrographic and elemental geochemistry studies on granites in the eastern segment of South Altun Mountains were carried out. The lithology of the area is mainly granite, granodiorite and monzonite, with SiO₂of 57.35% ~ 88.11%, Al₂O₃of 3.84%~19.56%, and total alkali content 1.00%~9.92%, with different degrees of Eu (Eu is 0.24~1.04), which belongs to the alkalic-alkali calcic-calc alkalic-calcic series, all of which have the characteristics of peraluminous, enrichment of large ion lithophile elements(LILE) and loss of high field strength elements(HFSE). Based on the results of previous studies and the classification of Sr-Yb, the Palaeo-mesozoic magmatism in this area can be divided into six episode: episode 1 (484-458 Ma) is the Nanling Mountains I-type granite, which may be derived from partial melts of metamorphic basalts; episode 2 (450~406 Ma) is Himalayan and Zhejiang mixed type granites, which have I-S and S type granites respectively, and the source areas are partial melts products of garnet amphibolite and metamorphic mudstone sandstone respectively; episode 3 (404~385 Ma) is the Nanling Mountains A2-type granite, which may be the partial melts product of the granodiorite at low pressure ; episode 4 (380~343 Ma) is the post-collisional uplifts of the Adakite and Zhejiang type granites, which are of type I granite and are related to the crustal detachment;and episode 5 (253~280 Ma) is the Nanling Mountains S type granites, It may be derived from the partial melts of metamorphic mudstone, which is the product of the closure of the Tethys Ocean; episode 6(154 ~ 230 Ma) is Himalayan I-type granite, and the source area is the partial melts product of meta-sandstone, which is related to subduction of residual ocean basin.

petrogenesis

South Altun orogenic belt

Palaeo-mesozoic

Qinghai-tibet plateau

As the most extensive type of rock on earth, granite has recorded rich information about crust-mantle interaction and different periods of orogenic belts in detail (Hawkesworth and Kemp, 2006; Wu FY et al., 2008; Lundstrom and Glazner, 2016; Wu CL et al., 2016; Chen GN et al., 2017; Zhang et al., 2021).Petrogenesis, geochemical characteristics, magmatic intrusion and formation mechanism, etc. (PEARCE et al., 1984; Creaser et al., 1991; Eby, 1992; Guo ZJ et al., 1998; Zhang JX et al., 1999; Chappell and White, 2001; Yu HF et al., 2002; Xiao FQ et al., 2006; Bonin, 2007; Li et al., 2007; Liu et al., 2007; Xu ZQ et al., 2006; Wu SP et al., 2007; Shannon et al., 2017; Wu CL et al., 2008; Wu CL et al., 2014; Kang L et al., 2016; Wang LS et al., 2016; Wu CL et al., 2016; Zhai MG, 2017; Wang XL, 2017;Chen HJ, 2018; Chen HJ et al., 2018) has always been a cutting-edge research topic.In recent years, the development of bedrock gas reservoirs in the eastern part of the Southern Altun Mountains was in full swing. However, the deep drilling depth and difficulty in coring, there is little information about the vertical and horizontal distribution and evolution of bedrock lithology in the area, and the genesis and origin of multi-episode intrusive granites are still weak, but some progress has been made(Jian KK et al., 2018; Ju YJ et al., 2018; Tao JY et al., 2018; Xi B et al., 2019). For example, According to Jian KK (2018), the monzogranite in The Datonggou area belongs to the high-k calc-alkaline series, and its source materials are mainly derived from metaglywacke and argillaceous rocks in the upper crust. The zircon age is (353.7 ± 1.1)Ma, which belongs to the early Carboniferous.Tao JY et al proposed that Dongping S-type granites were formed in the island arc environment of continental margin.The above studies are basically focused on a single region, and there is a lack of research on south China A systematic analysis of magma genesis and dynamic mechanism in the eastern Altun Mountains.Therefore, based on the previous experimental data and regional geological structure characteristics, this paper conducted a detailed geochemical study on the core samples of the eastern part of the Southern Altun Mountain, and studied the genesis and tectonic evolution of the granitic magma before the Altun Mountain to explore the relationship between.

The Altun orogenic belt is located in the northern margin of the Qinghai-Tibet Plateau. The main body of the Altun orogenic belt is formed by subduction and collision of blocks originating from The Gonwana continent in the Early Paleozoic. It has experienced many high-pressure and ultra-high-pressure effects caused by magmatic intrusion and plate convergence, and then was transformed by large strike-slip faults in the Meso-Cenozoic. It is a composite orogenic belt composed of multi-temporal and multi-geological bodies (Guo ZJ and Zhang ZC, 1999; Liu L et al., 1999).According to the existing geological data, the Altun orogenic belt can be divided from north to south into Abei metamorphic terrane (Dunhuang block), north Altun ophiolite melange belt, middle Altun block, South Altun high and ultra-high pressure belt (South Altun block), and South Altun Early Paleozoic ophiolite melange belt (Xu ZQ et al., 1999; Qin XF et al., 2006; Cao YT et al., 2013)(see Fig. 1a,b).

The study area is located in the eastern segment south of the Altun fault and consists of proterozoic crystalline basement, Paleozoic magmatic rocks and meso-Cenozoic sedimentary strata (Jiao XQ et al., 2020).The proterozoic strata consist of Dakendaban Group, Jinshuikou Group, Bashikurgan Group and Solkuri Group, and the lithology is mainly metamorphic rocks such as mica schist, gneiss and slate.In the early Paleozoic strata, the Ordovician is mainly composed of deep-sea sedimentary rocks and volcanic rocks.The upper Devonian and Carboniferous were developed in the Late Paleozoic with volcanic rocks and shallow Marine sedimentary rocks (Wu CL et al., 2008; Ma S et al., 2018).This study mainly focuses on the granite bodies in The Jianbei, Dongping, Niudong-Niuzhong and Datonggou areas, which are in intrusive contact with the Dakendaban Group and in unconformity contact with the overlying sedimentary rocks. The lithology includes granodiorite, granite and monzogranite, and the rock samples come from different depths of different Wells (See Fig. 1C,d).

The granite is gray, fleshy red, light red, massive structure, medium to fine semiidiomorphic crystal isomorphic structure, the main minerals are plagioclase (33%-36%), potassium feldspar (31%-33%), quartz (19%-30%), among which, the dark minerals in the north of the Jianbei area are mainly hornblende (4%-6%), biotite (3%-5%). The main dark minerals in Niudong-Niuzhong and Dongping areas are biotite (3%~10%), and the secondary minerals are zircon and apatite. The quartz is alloform and granular under microscope, and some biotite and amphibende are changed into chlorite. Granodiorite, are pale, block structure, the - fine grained half since the structure, main minerals are plagioclase (about 46%), potassium feldspar (about 26%), quartz (about 15%), dark minerals in hornblende (about 8%), biotite (about 5%) is given priority to, visible accessory minerals such as magnetite, apatite, a small amount of feldspar changed into sericite and kaolinite;The monzogranite is gray, red, massive structure, the main minerals are potassium feldspar (31%~45%), plagioclase (25%~34%), quartz (24%~26%), the dark minerals are mainly biotite (2%~7%), the secondary minerals are apatite and a small amount of zircon, etc.

The tests of rock powder samples and chemical complete analysis in Jianbei area were completed in Sichuan Keyuan Engineering technology Testing Center.The main elements were measured by XRF method and AXIOS X-ray fluorescence instrument.Rare earth elements were detected by plasma mass spectrometry with ICAP RQ plasma mass spectrometer.The analytical results all meet or exceed the relevant requirements of "Quality Management Standard for Laboratory Testing of Geology and Mineral Resources", and the element analysis accuracy is greater than 5%.

The geochemical test results are shown in Table 1.In addition, granites from Dongping, Datonggou and Niudong-Niuzhong area are systematically collected in this paper.Major and trace elements and chronological data were obtained from the following literatures : Jian KK et al., 2018; Xi B, 2019; Xin Y, 2020.

Table 1

contents of major (wt%) , trace and rare earth elements (× 10^-6) in samples

Samples	JB1-3	JB1-3	JB101		JB1-3	JB1-3	JB101
SiO₂	87.69	88.11	64.36	Rb	35.53	32.59	87.29
TiO₂	0.32	0.33	0.93	Sr	64.98	65.18	111.57
Al₂O₃	3.84	3.97	15.68	Zr	8.93	9.34	27.52
Fe2O3	1.73	1.75	2.06	Nb	1.99	2.03	19.25
FeO	0.90	0.95	2.97	Cs	0.78	2.10	0.78
FeO^*	2.46	2.52	4.82	Ba	195.58	180.91	725.90
MnO	0.08	0.09	0.08	Hf	0.30	0.27	0.80
MgO	0.95	1.08	2.36	Ta	0.06	0.06	0.77
CaO	0.88	0.79	1.57	Pb	7.46	10.40	7.12
Na₂O	0.80	0.69	4.98	Th	25.30	25.86	1.97
K₂O	0.98	0.85	2.16	U	0.59	1.78	0.61
P₂O₅	0.03	0.05	0.18	Y	1.77	1.91	7.94
LOI	0.66	2.98	0.63	La	41.19	48.86	8.24
TOTAL	98.03	97.12	98.48	Ce	71.54	75.48	16.30
A/NCK	1.21	1.43	1.31	Pr	8.94	9.31	2.43
A/NK	1.62	1.93	1.49	Nd	34.45	34.36	10.29
R1	5248.01	5343.23	1854.56	Sm	5.11	5.81	2.49
R2	216.62	215.99	592.67	Eu	0.50	0.56	0.77
CIPW Corundum	0	0.40	1.95	Gd	3.86	3.71	1.90
δ	0.07	0.05	2.39	Tb	0.36	0.40	0.34
Sc	1.17	8.47	1.12	Dy	0.68	0.72	1.86
Cr	66.41	92.58	69.24	Ho	0.08	0.08	0.31
Co	66.41	92.58	69.24	Er	0.37	0.39	0.70
Ni	4.28	9.18	3.90	Tm	0.02	0.02	0.10
Cu	5.76	8.56	5.41	Yb	0.16	0.16	0.62
Zn	34.46	70.17	35.31	Lu	0.03	0.03	0.10
Ga	8.23	8.21	18.47	∑REE	167.29	179.89	46.45

3.1 Jianbei Area

There are granodiorite and granite lithology in jianbei area.Granodiorite SiO₂ is 64.36%, Al₂O₃The content is 15.68%, Na₂O content is 4.98%, K₂O content is 2.16%, total alkali content is 7.14%, FeO* content is 4.82%, TiO₂Content is 0.93%;Granite the SiO₂Changes in 87.69%~88.11%, Al₂O₃The content is 3.84%~3.87%, Na₂O content is 0.69%~0.80%, K₂O content is 0.85%~0.91%, total alkali 1.23%, FeO* 2.46%~2.52% and TiO₂content is 0.32%~0.33%.In the SiO₂-(Na₂O+ K₂O) in the diagram (see Fig 2), the samples fall into granodiorite and granite respectively. The K₂O/ Na₂O value ranges from 0.43 to 1.23 (average 0.96), and the saturation index A/NCK value ranges from 1.21 to 1.43, indicating that the aluminum is peraluminized.The content of CIPW corundum in different rocks ranges from 0 to 1.95 (average 0.79), the Rittman index ranges from 0.05 to 2.39, and the basicity A.R ranges from 1.96 to 2.41, according to SiO₂-MALI map (See Fig 3). The rock is alkalic-alkali calcic-calcic series.As can be seen from the Harker diagram (see Fig 4), SiO₂With TiO₂, CaO, Al₂O₃, MnO, MgO, FeO*, P₂O₅that shows a good negative correlation, suggesting that ilmenite, garnet, apatite and other crystalline minerals may remain in the source region.

The total amount of REE in various granites varies widely, from 46.45×10^-6 to 167.29×10^-6 to 179.89× 10 in granodiorite and granite.The LREE/HREE values varied greatly (26.83~31.65, average 20.61), (La/Yb)_Nis 9.53~219.05 (average 101.44), (Gd/Yb)_N2.54~19.96(mean 10.54), (La/Sm)_NThe value ranges from 2.14 to 5.20(average 4.41), showing the right-leaning type of LREE enrichment and HREE relative loss (See Fig 5), indicating that LREE differentiation is higher than HREE differentiation. However, the REE distribution patterns of various rock masses are significantly different, suggesting that the Jianbei rock mass is composed of multiple emplacement granitic rocks.The granodiorites are characterized by depletion of high field strength elements Nb, Sr and Zr, enrichment of large ion lithophile elements Rb, Ba and K, and enrichment of Elements Nd and La (See Fig 6). Except for granodiorites, the granodiorites show positive Eu anomalies (1.04).In addition, the other samples showed obvious negative Eu anomalies (δEu ranged from 0.33 to 0.49, with an average of 0.41).

3.2 Dongping area

The lithology of Dongping area is mainly monzonite and granite.Monzogranite SiO₂Changes in 71.06%~77.28%, Al₂O₃The content is 11.88%-15.29%, Na₂O content is 0.75%~3.15%, K₂O content is 3.80%~5.69%, total alkali content is 4.55%~8.84%, FeO* content is 1.19%~1.87%, TiO₂The content ranged from 0.15% to 0.28% in SiO₂-(Na₂O+ K₂O) in the diagram (see Fig. 2), all samples fall into granite; The K₂O/ Na₂O value ranges from 1.32 to 5.07 (average 2.08), and the aluminum saturation index A/NCK value ranges from 1.17 to 1.76, indicating that the aluminum is peraluminized.The CIPW corundum content in different rocks ranges from 0.15 to 2.70 (average 1.25), the Rittman index ranges from 0.60 to 2.78, and the basicity A.R ranges from 1.51 to 2.53.SiO₂-MALI diagram (See Fig 3) shows that the rock belongs to alkalic -calc alkalic-calcic series.As can be seen from the Harker diagram (see Fig 4), SiO₂And K₂O, TiO₂, CaO, P₂O₅Negative correlation was not significant.The total amount of REE in monzogranite is 110.09×10^-6~ 173.46 x 10^-6, the total amount of rare earth in granite is 111.62×10^-6~ 200.23 x 10^-6, the variation range of the total amount of granite is larger.LREE/HREE values range from 9.22 to 14.27 (average 11.70), (La/Yb)_N11.95~19.35 (average 15.56), (Gd/Yb)_N1.48-2.14 (mean 1.85), (La/Sm)_NIn the range of 4.77~5.90(average 5.30), LREE enrichment and HREE flat distribution pattern of negative Eu anomaly (δEu range 0.43~0.62, average 0.54), LREE differentiation is higher than HREE. The granites are enriched in Rb, Ba, K large ion lithophile elements, and depleted in Nb, Sr, P, Ti high field strength elements (See Fig 5, 6), indicating that the granite has undergone the separation crystallization of ilmenite, apatite, plagioclase and other minerals.

3.3 Datonggou area

Datonggou monzogranite SiO₂Changes in 57.35%~74.51%, Al₂O₃The content is 13.57%~19.56%, Na₂O content is 2.94%~5.26%, K₂O content is 2.25%-6.42%, total alkali 6.22%-9.92%, FeO* 0.84%-5.45%, TiO₂content is 0.14%~1.20%.In the diagram of SiO₂-(Na₂O+ K₂O) (See Fig 2), the samples fall into monzonite, diorite, granodiorite and granite.The K₂O/ Na₂O value ranges from 0.47 to 1.85 (average 1.09), and the A/NCK value ranges from 1.06 to 1.26, indicating that the aluminum is peraluminized. The CIPW corundum content in different rocks ranges from 0 to 0.68 (average 0.25), the Rittman index ranges from 2.10 to 4.17, and the basicity ratio A.R ranges from 1.94 to 3.27₂- MALI diagram (See Fig 3) shows that the rock is alkaline, alkali-calcium-alkaline series;SiO₂ can be seen from the Harker diagramWith TiO₂, CaO, Al₂O₃, MnO, MgO, FeO*, P₂O₅that shows a good negative correlation (see Fig 4), and there may be residual crystal minerals such as ilmenite, garnet and apatite in the source region.

The total amount of REE in monzogranite varies from 106.25×10^-6to 502.69×10^-6, with LREE/HREE values ranging from 2.28 to 17.97 (average 9.24), (La/Yb)_Nis 1.86~27.77 (average 12.06), (Gd/Yb)_N0.79~2.50(mean 1.68), (La/Sm)_NThe value ranges from 2.06 to 9.69(average 4.41), which is similar to the Dongping pluton. Except for Y-9 REE distribution diagram, which shows "seagull type", the plutons all show LREE enrichment and HREE flat distribution pattern, which indicates that the Datonggou monzogranite has the characteristics of multi-episode intrusion, and the Y-9 pluton was formed in the late magmatic intrusion.The samples are characterized by negative Eu anomaly (δEu range 0.24~0.86, average 0.53), enrichment of Rb, Ba, K large ion lithophile elements and depletion of Nb, Sr, P, Ti high field strength elements (Fig.5, 6).

3.4 Niutong-Niuzhong area

Niudong-Niuzhong are granite, SiO₂ content are 66.83% and 74.70%, Al₂O₃ content are 13.89% and 13.36%, Na₂O are 4.40% and 3.21%, K₂O contents are 1.70% and 4.45%, total alkali contents are 6.10% and 7.66%, FeO* contents are 2.70% and 1.16%, The TiO2 contents are 0.37% and 0.14% respectively .in SiO₂-(Na₂O+ K₂O) in the diagram (See Fig 2), the samples fall into granodiorite and granite respectively;K₂O/ Na₂O value is 0.39, 1.39, aluminum saturation index A/NCK value is 1.11, 1.24, respectively, showing the characteristics of aluminum.The CIPW corundum content in the rocks is 0 and 1.30, the Rittman index is 1.56 and 1.85, and the basicity ratio is 2.69 and 2.82, SiO₂-MALI diagram (see Fig 3) shows that the rock belongs to alkali-calcium and calcium series.

The total amount of REE in Niudong and Niuzhong granites varies within a small range of 111.20×10^-6, 149.83×10^-6, LREE/HREE values are 10.56, 10.29, (La/Yb)_N Is 15.54, 9.78, (Gd/Yb)_N 2.23, 1.03, (La/Sm)_N 4.56,6.19,the δEu anomalies of samples are 0.79 and 0.29, respectively. The original mantle-normalized cobweb diagram of trace elements shows that the rocks are enriched in Rb, Ba, K large ion lithophile elements and depleted in Nb, Sr, P, Ti high field intensity elements(See Fig 5 and 6), indicating that the granite has undergone the separation crystallization of ilmenite, apatite, plagioclase and other minerals.

4.1 Petrogenetic types

Accurate classification of granite genetic types has always been the primary issue in the study of granites. At present, I-S-M-A classification scheme has become the mainstream and widely accepted by scholars. Common granite types are mainly I type, S type and A type (Ma HW, 1992; Chappell and White, 2001; FROST et al., 2001).Among them, I-type and S-type granites are distinguished by the nature of magma source area where granites were formed, while A-type granites almost include other granites except I-type and S-type granites, and their formation is related to crustal thinning events (Whalen, 1987; Eby, 1990, 1992; Patino Douce, 1997; Sylvester, 1998; Barbarin, 1999; Zhang Q et al., 2006b; Wu SP et al., 2007; Jia XH et al., 2009; Zhang Q et al., 2011; Zhang Q, 2014a).SiO₂ of the Southern-Altun front rocks range from 47.96%~88.11%, A/NCK value range from 0.97 to 1.76 (average 1.22), it is dominated by peraluminous rocks.According to diagram of (K₂O+Na₂O)/CaO-Zr +Nb+Ce+Y and FeO*/MgO-Zr+Nb+Ce+Y (See Fig. 7) shows that most of the rock samples fall into the I&S type granite area, and some of the samples fall into the differentiated granite area. It is generally believed that in the process of magmatic crystallization differentiation, Therefore, whole-rock Zr/Hf is used as A marker of granitic magma crystallization differentiation degree (Zr/Hf>55, common granite;25<Zr/Hf<55, moderately differentiated granites; Zr/Hf<25, highly differentiated granites) (Wu FY et al., 2017). The Zr/Hf of the Southern-Altun front rocks is 29.71-45.53, indicating that the rocks belong to moderately differentiated granites.Further use of ACF and Y-Rb(see Fig)I&S type granite discriminant chart shows (See Fig 8) that the northern ACF granodiorite and granite fall into S and I type granite in ACF diagram, and there is an obvious positive correlation in the Y-Rb diagram. The dark minerals are dominated by hornblende and biotite, combined with K₂O/Na₂O(0.43~1.23, average 0.83) and CIPW corundum (0~1.95, average 0.59), suggesting that the Jianbei granite is a type I granite and granodiorites are I-S transitional granite.In ACF diagram, Dongping granite and monzogranite all fall into S-type granite, and there is no obvious positive correlation in Y-Rb diagram, and no hornblende minerals are found K₂O>Na₂O and CIPW corundum (0.15-2.70, average content 1.19), indicating that Dongping granites meet the criteria of S-type granite.In the ACF diagram, all the Datonggou monzogranite bodies fall into the I-type granite, and the Y-Rb diagram shows a significant positive correlation, and CIPW corundum <1, which is consistent with the characteristics of I-type granite.In ACF diagram, Niudong granite body all falls into type I granite, and K₂O<Na₂O and no CIPW corundum content. Niudong granitic has the characteristics of I-type granite. The granite mass in Niuzhong falls into the S-type granite in the ACF diagram, K₂O>Na₂O with CIPW corundum content greater than 1, it is considered that Niuzhong granite is S-type granite.

Datonggou monzogranite (Y-5, Y-6, Y-7) falls into a-type granite area (See Fig 7), and Zr+Nb+Ce+Y content is 1052.35 ×10^-6, 393.49×10^-6, 520.19×10^-6, respectively. The lower limit of typical A-type granite is 350×10^-6. Experimental petrology shows that, A-type granites generally have higher melts temperatures (Whalen, 1987; Wang Q et al., 2000; Wu FY et Al., 2007), which can be used for Al₂O₃/TiO₂The ratio indicates the temperature of the slurry (Watson, 1979; Watson and Capobianco, 1981), when Al₂O₃/TiO₂>100, its melts temperature is less than 875℃, when Al₂O₃/TiO₂<100, its melts temperature is greater than 875℃, Datonggou rock Al₂O₃/TiO₂Respectively 16.30, 14.31, 17.42, indicating that the melts temperature is greater than 875℃.In addition, the Datonggou monzogranite is rich in silicon and alkali, poor in calcium, magnesium and aluminum, rich in Rb, Zr, Hf, Y, Ta, poor in Sr, Ba, Ti, P, and has obvious Eu negative anomaly and fall into the region of magnesium A-type granite in the discrimination diagram of FeO*/(FeO* +MgO)-SiO₂ (Whalen, 1987; Anderson, 1989);Eby classifies A-type granites into A1 and A2 subgroups (Y/Nb<1.2, A1 type granites;Y/Nb>1.2, A2 type granite) (Eby, 1990, 1992), Datonggou granites all fall into A2 type granite area,It belongs to magnesium A-type granite (see Fig 9), indicating that the rock mass is similar to the crust and island arc basalt, and is formed by magma derived from continental crust or subplate crust under the background of continental collision or island arc magmatism.

4.2 Rock source area

4.2.1 Jianbei Area

The Zr /Hf ratios of Jianbei granodiorites and granites range from 29.71 to 34.59, and the Sm/Nd ratios range from 0.15 to 0.24, which are similar to the crustal characteristics (~37, 0.17 to 0.25) (Taylor and McLennan, 1995). Mg# values ranged from 40.77 to 46.60, higher than the average value of mantle (40), indicating that the rock mass was derived from partial melts of the crust and mixed with mantle materials (RAPP and WATSON, 1995).Among them, the granodiorite is a sodium high Mg# (>40~45) granite (Zhang Q et al., 2010b; Xiong XL et al., 2011), chondrite normalized partitioning diagram of rare earth elements shows weak Eu positive anomalies, relatively flat HREE with low Sr (111.57×10^-6), low Y (7.94×10^-6) and low Yb (0.62×10^-6), belonging to Himalayan type granite with high pressure and low temperature (See Fig 10), with residual facies of plagioclase + hornblende + garnet occurring at 40~50km from the crust (See Fig 11) (Zhang Q et al., 2006a; Zhang Q et al., 2006b; Zhang Q et al., 2010b, 2010A; Zhang Q et al., 2011; Aircraft,2014 b, 2014 a).The granodiorite has a higher Nb/Ta ratio (24.88, 18~20) than the chondrite, indicating that the granodiorite is derived from high Nb/Ta Partial melts of hornblende and biotite minerals in the lower crust is one of the important ways to control the high Nb/Ta ratio (Tiepolo and Vannucci, 2014; Ballouard et al., 2016; Tang et al., 2019; Wang D et al., 2021; RL Rudnick., 2003).Based on multiple discriminant diagrams (see Fig 12) (Chappell, 1992; Douce and Harris, 1998; Sylvester, 1998; Patino Douce, 1999; Altherr et al., 2000), granodiorites at the diagram of CaO/Na₂O- Al₂O₃/TiO₂falls into the greywackes-derived melts, at the diagram of Al₂O₃/(FeO*+MgO+TiO₂)- Al₂O₃+FeO*+MgO+TiO₂ shows that the rock mass falls into the source area of amphibolite-derived melts.In the A/MF-C/MF diagram, the rock partial melts from metapelitic sources, and in the Rb/Ba-Rb/Sr diagram, the rock mass falls in the source region of pelitic-derived melt.Combined with the above main and trace elements discrimination, it is believed that the source rock may be garnet amphibolite with high Nb/Ta ratio formed by partial melts in the lower crust (Wang D et al., 2021).

The granite is a potassic high Mg# granite with extremely rich silicon and extremely low aluminum, and has a low content of FeO*+MgO+TiO₂ (average is 3.83%), suggesting that it may cause partial melts due to dehydration of water-bearing minerals (A. E. PatiNO, 2005).The chondrite normalized distribution diagram of REE elements shows that the rock mass has obvious negative Eu anomaly (δEu 0.33~0.34), heavy REE deficit is serious, and low Sr (64.98×10^-6~65.18×10^-6), low Y (1.77×10^-6~1.91×10^-6) and low Yb (0.16×10^-6), belong to theNanling type granites of low pressure and high temperature, similar to low-aluminum TTG, occurring in the thinning crust (<30km) (see Fig 10 and 11).The Rb/Sr ratio ranges from 0.50 to 0.55, and average Rb/Ba ratio is 0.18. In the diagram of Rb/Ba-Rb/Sr, the rock mass falls in the source of clay-poor (see Fig 12d), indicating the characteristics of continental crust remelts.Defant and Drummond point out that partial melts of the crust of low basalt intrusion and not too thick can form low aluminum TTG, and its residual calcium-rich plagioclase can inhibit Sr and Al₂O₃ in the melting.The results show that the lower crust (1.6GPa, 53km) high alumina basalts can form low alumina TTG (Al₂O₃The SiO = 12.3%₂=68%) by 5%~10% melts and the residual minerals through hornblende dehydration are plagioclase-monoclinopyroxene-plagioclyroxene (Defant and Drummond, 1990, 1993);In addition, at low pressure, the melt tends to be rich in Ca and poor in Na, while the source region K₂O lower the content is, the more melt tends to be granitic or granodiorite (Sisson et al., 2005; Martin and Sigmarsson, 2007), combined with the characteristics of high Nb/Ta ratio of the granites, it indicates that the granites are formed by poor K under low pressure environment and high Nb/Ta ratio metamorphic basalts were formed by partial melts induced by dehydrating amphibole.

4.2.2 Dongping Area

The Zr/Hf ratios of Dongping granite and monzogranite range from 30.54 to 35.44, and the Sm/Nd values range from 0.17 to 0.19, which are similar to the crustal characteristics (~37, 0.17 to 0.25). Mg# values range from 31.64 to 45.39 (mostly lower than mantle 40) (Taylor and McLennan, 1995), with low FeO*+MgO+TiO₂.It indicates that the granites may have partial melts of the crust due to the dehydration of water-bearing minerals, and the granites as a whole appear to be silica-rich, aluminum-poor and potassium-poor, low Mg# (<40~45) with obvious negative Eu anomaly. The chondrite distribution diagram of HREE shows relatively flat, suggesting that the partial melts is related to amphibolite facies.According to Sr-Yb classification mark, granite and monodiorite can be divided into Himalayan type granite and Zhemin type granite (DPH301 (see Fig 10a)) (Zhang Q et al., 2006a; Zhang Q et al., 2006b; Zhang Q et al., 2010b, 2010A; Zhang Q et al., 2011; Zhang Q, 2014B, 2014a), the difference lies in the melts pressure and temperature in the source region. The residual minerals of Zhemin type type are plagioclase+ amphibolite, the pressure is usually < 0.8GPa, and the temperature is 900℃~1000℃.Himalayan type residual minerals are composed of plagioclase + hornblende + garnet. The pressure is usually > 1.0GPa and the temperature is 750℃~900℃ (see Fig 10b).Combined with multiple discriminant diagrams (see Fig. 12), Himalayan type granite in diagram of CaO/Na₂O-Al₂O₃/TiO₂, it falls into the region of greywackes-derived melts, and in diagram of Al₂O₃/(FeO*+MgO+TiO₂)- Al₂O₃+FeO*+MgO+TiO₂ is located in the region of greywackes-derived melts.In diagram of A/MF-C/MF, the rock mass falls into the Partial melts from metapelitic sources, and in Rb/Ba-Rb/Sr diagram, the rock mass falls into the source region of pelitic-derived melt.In the diagram of CaO/Na₂O-Al₂O₃/TiO₂, Zhemin type granites fall into the melts source of greywackes-derived melts, and diagram of Al₂O₃/(FeO*+MgO+TiO₂)-Al₂O₃+FeO*+MgO+TiO₂ are located in the region of greywackes-derived melts. In the diagram of A/MF-C/MF, the rock mass falls into the Partial melts from metapelitic sources, and in diagram of Rb/Ba-Rb/Sr, the rock mass falls into the source region of pelitic-derived melt.The Nb/Ta ratio is further used to indicate the characteristics of magma source area. The Nb/Ta ratio of Himalayan type granite and Zhemin type granite is close to that of continental crust (11.77-13.67, 8-14) (RL Rudnick., 2003).It is concluded that the Himalayan-type granites are derived from partial melts of continental crust metamorphic sandstone and mixed with a small amount of mantle-derived materials, while Zhemin type granites are mainly derived from partial melts of metamorphic argillaceous sandstone.

4.2.3 Datonggou Area

The ratios of Zr/Hf, Sm/Nd and Nb/Ta in the Datonggou monzogranite range from 37.34 to 41.53, 0.14 to 0.30 and 5.88 to 12.76, which are similar to the crustal characteristics (~ 37,0.17 to 0.25, 8 to 14). Mg# values range from 27.93 to 47.20 (mostly lower than mantle 40), indicating that the rock mass is mainly derived from partial melts of the crust and may be mixed with mantle source.According to the Sr-Yb classification mark, it can be divided into Nanling type granites (Y-5, Y-6, Y-7), Zhemin type granites (Y-4, Y-8, Y-9) and Adakite type granites (Y-3) (See Fig 10a).Nanling type granite is A-type potassic-poor low Mg# (27.93-40.53) granite,which rich in silicon, aluminum-poor, calcium-poor and obvious negative Eu anomaly. The formation pressure is lower (< 0.8GPa) (see Fig 10b) and the temperature is higher. The residual minerals are mainly calcium-rich plagioclase. In the source discriminant diagram, it falls into the melts zone of greywackes-derived melts and pelitic-derived melt, showing the characteristics of continental crust remelts.According to Patino Douce(Patino Douce, 1997; Douce and Harris, 1998; Patino Douce, 1999) showed that calc-alkalic tonodiorite and granodiorite could be dehydrated and melted by biotite at low pressure and high temperature (< 0.8GPa, T=950℃) to produce A-type granite, and its residual phase was at least plagioclase + monoclinopyroxene (< 0.4GPa) at relatively low pressure. Under relatively high pressure force, at least for the plagioclase + enstenite, through comparative analysis, chase ditch Nanling A-type granite and granodiorite melts characteristics are very similar, indicating that chase ditch Nanling A-type granite is likely to be under the condition of low pressure end of basaltic magma invasion caused granodiorite (< 20%, melts degREE partial melts depth < 30 km) form.

Adakite type granites are Medium-type high-Mg # granites with moderate silica-rich, aluminum-rich and weak Eu negative anomalies (See Fig. 10). Which belong to Type C Adakite type granites, which are also comparable to type Ⅱ Adakite type granites classified by Wang Q et al., 2001; Zhu DC et al., 2002; Zhang Q et al., 2003; Xiong XL et al., 2011; Zhang Q, 2011, which is due to the melts of the lower crust caused by crustal thickening (continental crust >50Km), rather than the result of plate subduction. The original mantle-normalized cobweb diagram of trace elements shows that the Adakite has significant negative NB-Ta anomaly, and the Nb/La ratio is lower than the average value of lower crust rocks (0.32, 0.6), and close to the Nb/La ratio of lower crust rocks (granulite xenoliths, granulite terrima and Precambrian basic volcanic rocks) (Xiong XL et al., 2011). The rock formation pressure is >1.5 GPa (residual rutile), suggesting that the Adakite may be mafic or metamorphic mafic rocks formed by partial melts, while K₂O is related to the source area, which suggests that the Adakite may be a metamorphic medium and high potassium metamorphosed medium basic rock partially fused under high pressure to work out.Zhemin type granites are silicon-poor, silicon-rich, aluminum-poor, high-k calc-alkaline, sodium and low-Mg # granite with obvious negative Eu anomaly, formed at low pressure(< 1.0GPa), medium and low temperature environment, HREE is relatively flat, in equilibrium in amphibole facies (see Fig 10), in the diagram of CaO/Na₂O- Al₂O₃/TiO₂Falling into the greywackes-derived melts and falls into the source of amphibolite-derive in the diagram of Al₂O₃/(FeO*+MgO+TiO₂)-Al₂O₃+FeO*+MgO+TiO₂.In A/MF-C/MF diagram, it falls in the partial melts from metagreywackes sources.In the Rb/Ba-Rb/Sr diagram, it falls in the clay-poor and clay-rich melt zone (see Fig 12).Based on petrography, major and trace elements, it is concluded that the Zhemin type type rock mass originated from partial melts of amphibolite facies in the lower crust.

4.2.4 Niudong-Niuzhong area

The ratios of Zr/Hf, Sm/Nd and Nb/Ta are 30.43~31.90, 0.13~0.18 and 7.42~7.49, which are similar to the crustal characteristics (~ 37,0.17 ~0.25, 8~14).Among them, the Niudong granite is a moderately silica-rich and aluminum-poor sodium high Mg# granite. The chondrite normalized distribution diagram shows that the granites have weak negative Eu anomalies, and HREE is relatively flat with low Sr (220.20×10^-6) and low Y (14.27×10^-6) and low Yb (1.16×10^-6), belonging to Himalayan type granites (see Fig 10a), with higher pressure and lower melts degree (Sr increase) than Himalayan type granites in Jianbei and Dongping area (Zhang Q et al., 2010b; Zhang Q et al., 2011; Zhang Q, 2014a);According to the cobweb diagram of trace elements, the granite has obvious negative NB-Ta anomaly and low Nb/La ratio (0.23), suggesting that there is a small amount of rutile residue in the source region, indicating that the formation pressure of the rock mass may be close to eclogite facies. Combined with various discriminant diagrams (see Fig 12), Monzogranite in the diagram of CaO/ Na₂O-Al₂O₃/TiO₂, it falls into the region of greywackes-derived melts, and is located in the diagram ofAl₂O₃/(FeO*+MgO+TiO₂)- Al₂O₃+FeO*+MgO+TiO₂and A/MF-C/MF, the rock mass falls into the partial melts region of alternation sandstone, and in Rb/ Ba-Rb /Sr diagram, the rock mass is close to the basalt source region, indicating that the rock mass may be caused by the partial melts of crustal metamorphic greywacke caused by the low intrusion of basaltic magma, and mixed with the rising mantle-derived magma.

Niuzhong granite is silica-rich and aluminum-poor potassic low Mg# granite with low FeO*+MgO+TiO₂. The chondrite normalized distribution diagram of REE elements shows that the rock mass has obvious negative Eu anomaly, HREE is relatively flat and has low Sr (42.60 ×10^-6), high Y (21.76×10^-6) and high Yb (2.51×10^-6), belonging to Nanling type granite with low pressure and high temperature, which is in equilibrium with hornblende and occurs in thinning crust (<30km) (see Fig 10).Granite in CaO/ Na₂O-Al₂O₃/TiO₂ diagram falls into the zone of Felsic-pelite-derived melts. In the diagram of Al₂O₃/(FeO*+MgO+TiO₂)-Al₂O₃+FeO*+MgO+TiO₂ ,the diagram shows that the granite falls into the source area of greywackes-derived melts, and the Rb/ Ba-Rb /Sr diagram falls into the clay-rich melts area (see Fig 12). Comprehensive studies suggest that the Niuzhong granite may be formed by the partial melts of crustal metamorphic argillaceous rocks triggered by the low intrusion of mantle-derived magma through dehydration of water-bearing mineral biotite.

4.3 Geological Significance

In the discriminant diagram of tectonic environment (see Fig. 13 and 14), except for the Datonggou A-type granite falling into the inner ring of the post-orogenic within plate, the other granites in different periods in the area all fell into the arc-related continental margin and post-collision region, among which the Jianbei granite fell into the oceanic arc region.The granitic rocks in Dongping and Niudong-Niuzhong area all fall in the volcanic arc granite area related to the collision.The Jianbei granodiorite falls into the late-orogenic region;The Datonggou I type granite falls into the volcanic arc region post-collision uplift.Previous studies have shown that the residual ocean basin in the southern margin of the South Altun was extended again in the early Paleozoic, forming the palaeogeographic features of the multi-island ocean. Then, the Late Ordovician Caledonian movement caused the Collision between the Qaidam block and the unified paleocontinent formed by the Tarim block and the Middle Altun microcontinent, leading to the closure of the South Altun Ocean. Thus the tectonic evolution of continent-continent subduction collision orogeny began (Guo ZJ et al., 1998; Xu ZQ et al., 1999; Qin XF et al., 2006).Song SG et al(song SG et al., 2007) believed that the subduction of oceanic crust should be greater than 460Ma based on olivine studies in the northern margin of Qaidam Basin. Wu et al. (Wu et al., 2018) believed that the orogenic belt was invaded by a large amount of magma floor after the early subduction of oceanic crust was broken off at 455Ma. The partial melts of the crust resulted in the formation of S-type and I-type granites, which were extended due to the equilibrium adjustment between different blocks. The tectonic environment of the granites changed from oceanic island arc to active continental margin and then post-collision.This indicates that the South Altun Mountain has been in a complex structure since the early Paleozoic Magmatic active period.

Zircon, isotopic age and geochemical characteristics of some rock masses obtained from previous studies (Wu SP et al., 2007; Wu CL et al., 2008; Wu CL et al., 2016; Wang N et al., 2017; Chen HJ et al., 2018; Jian KK et al., 2018; Tao JY et al., 2018; Xi B, 2019; Xi B et al., 2019; Jiao XQ et al., 2020; Xu N et al., 2020; Yang X, 2020), summarized six periods of granitic magmatic activity in the eastern part of the Southern Altun Mountains: The Nanling I-type granite formed in the first period of magmatic activity (484-458Ma) has oceanic arc characteristics, and the tectonic environment is island arc granite, indicating that the diagenetic age may be related to the early episode of oceanic crust subduction, and the magma originated from the thinned crust.The second episode (450~406 Ma) is the Himalayan I-S type and Zhemin S type volcanic arc granites, indicating that the Altun Ocean was completely closed, the orogenic belt entered the continental collision episode, and the crust thickened.The third episode (404~385 Ma) is Nanling A2 type granites with intraplate environment. With the readjustment of different blocks, the crust is in post-orogenic stretching and thinning state.The fourth episode (380~343 Ma) is the adakite and Zhemin type granite post-collisional uplift, with I type granite attributes, and is the product of subduction and compression of the Kunlun Ocean under the influence of tethys structure into the Qaidam block, and the crust was degenerated.During the fifth period (253~280 Ma), the Nanling S-type granitic magmatic activity was in the paleoTethys ocean closed evolution episode, during which the stress in the region changed from tension to compression, and the intracontinental subduction produced a large number of syn-collisiona S-type granites, such as Niuzhong granite.The formation of the sixth episode (154~230 Ma) Himalayan syn-collisiona I-type granites indicates that the residual ocean basin in the Zongwulong area was oblique subducted under the influence of the Indosinian movement. Compared with the fifth episode, the crust thickened significantly at this time.Combined with the outcropping location of granitic rock mass, the magmatic age of the eastern part of the Southern Altun Mountains gradually decreases from west to east.

(1) The granitic rocks in the area are rich in silicon and magnesium, rich in Rb, Ba, K large ion lithophile elements and deficient in Nb, Sr, P, Ti high field strength elements, with different degree of negative Eu anomaly.According to the geochemical characteristics of the rocks, the Jiabei granodiorite belongs to the alkali-calcic series, the granites belong to the peraluminous calcium series with the chemical properties of I-type island arc volcanic rocks, and the Jianbei granodiorite belongs to I-S type granite. The Dongping rock assemblage is monzonite + granite, which has the characteristics of S-type granite.The Datonggou Y-5, Y-6 and Y-7 monzogranite are A2 type granites, while the Y-3, Y-4, Y-8 and Y-9 monzogranite have the geochemical characteristics of I-type granites.Niudong and Niuzhong granites are I and S type granites.

(2) according to the classification of Sr-Yb content, the Jianbei granodiorites are Himalayan type granites with high Mg# high pressure and low temperature. The source rocks may be the metamorphic garnet amphibolites with high Nb/Ta ratio in the lower crust. The Nanling-type granites with low pressure and high temperature were formed by partial melts under low pressure, potassium-poor and high Nb/Ta ratio metamorphic basalts were formed by partial melts induced by dehydration of amphibole.The Himalayan-type granites in Dongping area are derived from partial melts of continental crust metagreywackes and mixed with a small amount of mantle-derived materials, while the Zhemin type granites are mainly derived from partial melts of metamorphic argillaceous sandstone.Datonggou Nanling type A granite area is likely to be under the condition of low pressure end of basaltic magma invasion caused granodiorite (< 20%, melts degree partial melts depth < 30 km) to form, Adakite may be metamorphic rock, high in potassium in metamorphic rock under high pressure environment formed by partial melts, zhemin type source rock mass under amphibolite facies of partial melts of crust;The Niudong Himalayan type granite may be formed by partial melts of crustal metamorphic greywacke caused by low basaltic magma intrusion and mixed with rising mantle-derived magma.Niuzhong Nanling type granites may be formed by mantle-derived magmatic low intrusion via aqueous mineral biotite Dehydration causes partial melts of crustal metamorphic argillaceous rocks.

(3) Based on the zircon, isotopic age and geochemical characteristics of some granitic rocks obtained by previous studies, the granitic magmatic activity in the area can be divided into six episodes: the first episode is 484 − 458 Ma, the second episode is 450 − 406 Ma, the third episode is 404 − 385 Ma, the fourth episode is 380 − 343 Ma, and the fifth episode is 253 ~ 280 Ma, and the sixth phase was 154 ~ 230 Ma

all authors (YY,CC,WL,,YJ, AX,HD,YY,XL)declarations:

thical approval was obtained from School of Geoscience and Technology, Southwest Petroleum University
I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper
Consent to Participate & Consent to Publish
Credit author statement: Yu YE: Data curation, Writing- Original draft preparation, Resources, Formal analysis, Writing- Reviewing and Editing.Chao CHENG:Conceptualization, Methodology,Edit, Project administration.Wusheng LI: Data curation, Resources.Guojun YANG: Data Curation.Aihua XI: Data Curation.Hongbing DENG: Data Curation. Songyu YANG: Data Curation. Xi Li: Data curation, Resources.
All data generated or analysed during this study are included in this article, and the datasets used or analysed during the current study are available from the frist author on reasonable request
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No competing interests reported.

Genesis and geological significance of Granitic Rocks in Eastern South-Altun Orogenic Belt

Status:

Version 1

Abstract

Figures

0. Introduction

1. Regional Geological Background And Sample Characteristics

2. Methods Of Petrogeochemical Analysis

3. Petrogeochemistry

4 Discussion

5 Conclusion

Declarations

References

Additional Declarations

Status:

Version 1