Characteristics And Genesis Analysis Of The Huatagou Complex Rock Mass In Changping,Beijing

The Huatagou complex pluton is located in Changping District,Beijing,geographically located in the east of the North China Plate.Due to the downward subduction of the Pacific Plate,magmatic activity in this area has been very strong since the Indosinian period,especially the Badaling Granite Complex.As a representative,the intrusive age of the rock mass in this area is mainly from the Early Jurassic to the Early Cretaceous,while the rock mass in the study area is formed by the upward intrusion of magma from the Late Triassic.The rock mass in the study area is composed of two parts:the inner ring granite and the outer ring syenite.The granite has a fine-grained isogranular structure.The main minerals are quartz(40%?45%),K feldspar(30%?35%),plagioclase(15%),and some biotite(about 7%),amphibole(3%).it is medium-coarse unequal grain structure and massive structure.The main rock-forming minerals are alkaline feldspar,plagioclase,biotite,and amphibole.There are signs of thickening of the grain size from the inside to the outside of the outer ring of syenite.In addition,the dark minerals near the center of the rock mass are mainly biotite,with less hornblende content,while the samples close to the outer side of the rock mass are significantly more hornblende,with no or a small amount of biotite.The inner ring granite belongs to the series of weakly peraluminous Mg high-potassium calc-alkaline rocks,enriched in light rare earth(LREE),depleted in heavy rare earth(HREE),and enriched in large ion lithophile elements such as Rb,Ba,Th,U,and Pb.,Loss of high field strength elements Nb,Ta,Zr,Hf and Sr,P,Ti,(⁸⁷Sr/⁸⁶Sr)_tvalue is0.70520?0.70566,(¹⁴³Nd/¹⁴⁴Nd)_tvalue is 0.51153?0.51159,?Nd(t)value-16.65?-15.19.The10000Ga/Al value of granite is 2.49-2.56,and the contents of P₂O₅,Na₂O and TFe O are0.04%?0.07%,3.49%?3.94%,and 1.75%?2.15%.The outer ring syenite is a high-potassium alkaline rock rich in Mg,which is rich in Rb,Sr,Ba,Pb,heavy rare earth(LREE)and other large ions Qinshi elements,and is relatively deficient in Nb,Ta,Th,U,Zr,Hf And P,Ti and other large ion lithophile elements.The ⁸⁷Sr/⁸⁶Sr value of syenite is 0.70543?0.70609,the ¹⁴³Nd/¹⁴⁴Nd value is 0.51188?0.51192,the(⁸⁷Sr/⁸⁶Sr)_tvalue is 0.70441?0.70496,the(¹⁴³Nd/¹⁴⁴Nd)_tvalue is0.51175?0.51179,?Nd(t)The value is-12.11?-11.36.Both were formed in the Early Jurassic,and their LA-ICP-MS zircon U-Pb dating results are both about 205±1 Ma.The?Hf(t)value of granite is-15.62?-7.70,and the age of the second-stage Hf model is 2.25 Ga?1.75 Ga.The?Hf(t)value of syenite is-8.41?-6.42,the age of the first-stage model is 1.19 Ga?1.12 Ga;and the age of the second-stage Hf model is 1.75 Ga?1.65 Ga.The syenite magma originated from the mantle-derived potassium-containing basic magma formed by the partial melting of the enriched mantle.It is formed after contaminating and separating and crystallization of crustal materials.After the first pulsation of the magma,it emplaces to form the outer ring of syenite.At the same time,the syenite magma further assimilates the crustal surrounding rocks and undergoes crystallization separation and evolution to form acidic granitic magma.After the second pulsation,the magma emplaces into the unconsolidated syenite to form an inner ring of granite.The genetic type of granite is A1 type granite,formed in the post-orogenic extensional structure background.