Intrusive formations
F. Kh. Apayarov

Intrusive formations occupy near 2600 km2, i.e. 43% of the mapped area. Late Ordovician granitoids are the most abundant.

Alamedin Formation (O₁a). Zakharov (1981) identified and described this formation. The type body occupies 120 km2 in the northern part of the map, on the northern slope of the Kyrgyz Range, in the basins of the Alamedin, Kara-Bulak and Uzun-Kur rivers. The Alamedin Formation encompasses 3 intrusion stages.
Stage 1 (δ, qδ₁ O₁a) is represented by inequigranular, medium-grained quartz diorites, diorites and rarely quartz monzodiorites. They locally display magmatic fabrics and occupy about 25 km2 as xenoliths and large bodies in the eastern part of the massif.
Stage 2 (δγ, γ₂ O₁a) is dominant (90 km2). Rocks are light-gray to pinkish-red medium-grained, locally porphyric biotite-hornblende granodiorite with subordinate amounts of quartz monzonite and granite.
Stage 3 (γ₃ O₁a) produced small, irregularly shaped bodies of light-pink, fine-grained biotite leucocratic granite covering 12 km2.
Petrochemistry. The SiO2 content varies from 53.9 to 72.9%; the Na2O+K2O content varies from 3.45 to 8.42%. Stage 1 and 2 rocks belong to the potassic-sodic (K2O/Na2O = 0.5-1.2) calc-alkali series; stage 3 granites belong to the potassic-sodic (K2O/Na2O = 1.1-1.9) subalkaline series. 
Age. These rocks intrude Upper Riphean sediments of the Karakorum Formation and Lower Ordovician sediments of the Djelamysh Formation. They are intruded by granitoids of the Issykata Formation. To the west of the map area, Upper Ordovician, Karamoynok Formation sediments unconformably cover Alamedin plutonites. Seven K/Ar ages on biotite and amphibole of stage 1 and 2 rocks (519 to 458 Ma, isotope age catalogue) were used to infer an intrusion age of 486 ± 9 Ma, inconsistent with the geological information. U/Pb dating on quartz monzonite (Sample 177al) yielded scattered ages (207Pb/206Pb=448, 207Pb/235U=367, 206Pb/238U =354 Ma) that do not identify the age of the rock. Although the technique is commonly regarded as unreliable, the information to assign the Early Ordovician age of these granitoids comes from numerous K/Ar ages of Alamedin rocks within and outside the map (Morozov, 1975). 

Kazyk Formation (ν, δ O₂k). I.L. Zakharov (1992) defined this formation of variably large (several m2 to several km2) xenoliths in granitoids of the Late Ordovician Suusamyr granitoids and one body on the southern slope of the Djumgal Range, near the eastern border of the map. Two 4 km2 big bodies occur in the Ichke-Suu – Archalytor interfluve and in the western part of the map, in the Djindisu – Irisu interfluve. Rocks vary from gabbro monzogabbro, to quartz diorites the more basic rocks being usually typical for larger xenoliths.

Petrochemistry. The SiO2 content varies from 43.5 to 60.6%; the Na2O+K2O content from 3.46 to 7.52%. The rocks belong to the potassic-sodic and potassic (K2O/Na2O = 0.62-1.25) subalkaline series. 

Fig.2.3 The Kazyk Formation rocks in the SiO2 – Na2O+K2O diagram (Bogatikov et al., 1987)

Age. Two sites of diorite xenoliths were analyzed for U/Pb and K/Ar dating. In one site, U/Pb dating on zircon yielded 409±5 Ma and K/Ar dating on biotite yielded 493±10 Ma (isotope age catalogue). This discrepancy suggests that at least the K-Ar isotope system has been altered. The 449±15 Ma zircon U/Pb age obtained from another site (isotope age catalogue) is preferred as intrusion age (Middle Ordovician) of this formation.

Fig. 2.4: Kazyk Formation rocks in the SiO2 ‑ K2O/Na2O

 diagram (Bogatikov et al., 1987)

Suusamyr Formation (O₃s). Rocks of this formation compose large plutons trending latitudinally in the North Tien Shan over 600 km (Osmonbetov, 1982). The Suusamyr Pluton occupies nearly 900 km2 in the southern part of the map. Two stages are separated.
Stage 1 (δγ, γ1 O₃s) is dominant and comprises porphyric granodiorites, granites and more equigranular, medium and large-grained granites.
Stage 2 (γ2 O₃s) is minor and comprises fine, medium-grained leucocratic granites composing small (< 2 km2) stocks, dike-like bodies and veins, except for one large (about 17 km2) granite on the left bank of the West Karakol River.
Petrochemistry. The SiO2 content varies from 62.0 to 75.0% and the Na2O+K2O content from 6.46 to 8.72%. Stage 1 granitoids plot in the SiO2 ‑ Na2O+K2O diagram (Bogatikov et al., 1987) along the boundary between calc-alkali and subalkaline series. Stage 2 granites belong to the subalkaline series.
Age. Suusamyr Granitoids intrude Upper Ordovician sediments of the Karamoynok Formation and are in their turn intruded by Late Silurian leucocratic granites of the Alama Formation. Isotope ages from the mapped area (isotope age catalogue) are abundant. Six K/Ar ages range from 383 to 444 Ma; 3 of them appropriately yielded 436 ±10, 436±15 and 444±10 Ma. Ten U/Pb ages range from 395 to 460 Ma, with five in the 433-449 Ma interval. At least one sample (No 2274) seems valid because various ratios of different uranium and lead isotopes gave similar ages such as 427±8, 429±8 and 437±5 Ma (isotope age catalogue). The upper intercept of 5 discordant points with concordia is interpreted as intrusion age at 436±14 Ma (Latest Ordovician; Fig.2.3.).

Fig.2.5. Intersection of the discordia and concordia, calculated by K. Ludwig’s method (1980).

Issykata Formation (O₃i). This formation occurs in the near-watershed and on the northern slope of the Kyrgyz Range as a series of 20 km2 to 450 km2 massifs (Rubtsov 1988). The Issykata Massif outcrops along the eastern border of the map, in the basins of the Issyk-Ata and Tuyuk rivers. The massif includes 3 stages.
Stage 1 (νδ, δ₁ O₃i) occurs in up to 2 km2 xenoliths of amphibolitized gabbros, gabbro-diorites and diorites, rarely quartz diorites and monzonites in stage 2 granitoids. In the northwestern part of the Issykata Massif a 18 km2 gabbro-diorite intrudes the Upper Riphean rocks of the Karakorum Formation. 
Stage 2 (δγ, γ₂ O₃i) is dominant (90% of the massif area) and is represented by medium, large-grained porphyric, usually foliated hornblende-biotite, biotite granodiorites and, in some places, plagiogranites.
Stage 3 (γ₃ O₃i) comprises fine, medium-grained granites and leucocratic granites forming small (< 2.5 km2) bodies, and aplites and aplitic granites forming dikes in stage 2granitoids.
In other occurrences, stage 2 granitoids usually dominate over up to 3-5% of stage 3 granitoids.
Petrochemistry. The SiO2 content of stage 1 rocks is from 47.34 to 63.42%, and in stage 2 granitoids it is from 63.26 to 73.10%. The Na2O+K2O content varies from 3.24 to 7.22% in the gabbros, diorites and quartz diorites, and from 5.62 to 9.10% in the granitoids. In the SiO2 ‑ Na2O+ K2O diagram, rocks plot along the line separating the calc-alkali and subalkaline series. In stage 1 rocks the K2O/Na2O ratio varies from 0.32 to 1.14; it is nearly 1 in stage 2 granitoids, except plagiogranites.
Age. The Issykata rocks intrude granites of the Alamedin Formation, Upper Ordovician sediments of the Chonkaindy and Djartash Formations and are intruded by Late Silurian leucocratic granites. Three U/Pb ages on zircons from stage 2 granitoids of the Issykata Massif yielded 418±8, 419±8 and 450±10 Ma (isotope age catalogue), consistent with the geological constraints.

Alama Formation (lγ S₂a). Leucocratic granites of the Alama Massif, on the northern slope of the Kyrgyz Range, 23 km to the west of the mapped area, were defined as typical rocks of this formation (Zakharov, 1981). They form 3 massifs and several small bodies spatially related to Late Ordovician granites.
Practically all plutons are composed of pinkish-reddish to red leucocratic fine, medium, rarely large-grained granites. They often look like granites of the previous intrusions and may be incorrectly identified as Silurian rocks. 
Petrochemistry. The SiO2 content ranges from 73.5 to 75.16% and the Na2O+K2O content from 7.96 to 8.82%. These granites belong to the subalkaline series (K2O/Na2O = 0.86-2.24).
Age. The Alama granites intruded Upper Ordovician sediments and granitoids of the Issykata Formation. K/Ar ages on biotite from the North Sokuluk Massif (Ala-Archa River) yielded 407±15 Ma (isotope age catalogue, No5051). U/Pb dating on zircon from the Alama Pluton yielded 390±10 Ma (Zakharov, 1992). Nine U/Pb zircon ages date the Kumyshtag granite of this formation, in the Talas Range, in the southwestern part of the map, at 410±10 Ma by (Apayarov, 2002).

Sandyk Formation (μ, ξ, φξ C₃-P₁s). The typical Sandyk Massif (75 km2) is located in the eastern part of the Djumgal Range, on the right bank of the East Suek River (Zlobin, 1960). Its northern part outcrops on the map. It is mainly comprised of dark-gray and gray biotite-diopside monzonites and syenites intruded by small bodies and stocks of light-gray and white nepheline syenites. Dikes and small bodies of syenite-porphyries, monzonite-porphyries and nepheline syenite-porphyries represent late magmatic phases (Zlobin, 1960). The Salyk Massif (10 km2), in the Alamedin River basin, in the near-watershed part of the Kyrgyz Range, is composed of monzonites, syenites and quartz syenites accompanied by dikes of syenite-porphyries and quartz syenite-porphyries (Zakharov, 1992).
Up to 50 m thick and up to 500 m long dikes of syenite-porphyries and quartz syenite-porphyries occur separately, usually controlled by fractures.
Petrochemistry. Two series are distinguished: a potassic-sodic (K2O/Na2O = 1,0-2,2) subalkaline and a potassic (K2O/Na2O = 1,8-6,0) alkaline. The SiO2 content in the subalkaline rocks varies from 55.0 to 66.5%, and in the alkaline rocks from 53.5 to 60.0%.
Age. K/Ar ages on biotite from nepheline syenites (295±10, 303±10, 304±10 Ma) and U/Pb zircon ages on syenites (294±8 Ma) (isotope age catalogue) correspond to the Carboniferous - Permian boundary.

Permian dikes (δπ, χ P). Various lamprophyres (kersantites, odinites, malchites, minettes), alkaline gabbroids and rarely diorite-porphyries occur practically all over the Central Tien Shan territory, locally forming dike swarms (Knauf et al., 1980). Within the map they occur in the Sokuluk-Djelamysh interfluve and on the right bank of the West Karakol River. Dikes are sub-vertical. Their thickness is usually 1-2 m, rarely 10-20 m; the length varies from a few tens to hundreds meters. They intrude all Paleozoic rocks including the Sandyk syenites. Although they are assigned a Permian age, some lamprophyres may be Mesozoic as suggested by K/Ar isotope ages (170-230 Ma) of similar rocks in other Tien Shan regions (Knauf et al., 1980).

Edited by J-P. Burg 
SNF, Project No 7KSPJ065518; http://www.kyrgyzstan.ethz.ch