A.E. Izokh

Gabbro-granite intrusive series and their indicator importance for geodynamic reconstructions

One of the problems faced by researchers when paleogeodynamic reconstructions are carried out for deeply eroded orogenic terranes is the limited usage of geological and isotopic geochemical data on volcanic associations. The utilization of information on gabbroids and granitoids considered separately also fails to resolve this problem. The convergence of features of arc, collisional, and within-plate magmatic processes leads the researcher to search for indicator plutonic associations, such as paired gabbrogranite intrusive series. The latter were distinguished using geoinformation databases (including those compiled by the authors of this paper), which were composed for the Early Caledonides in the Altai-Sayan folded area and adjacent territories. This makes it possible to characterize mantle-crustal magmatism in suprasubductional and collisional environments and the conditions under which these geodynamic regimes (plate- and plume-tectonic factors) interact. This paper presents estimates of the composition of the parental basic magmas, distinctive features of their differentiation, and the compositional specifics of the accompanying extensive granite-forming processes. The example of the Altai-Sayan folded area and adjacent territories is employed to correlate the composition of the basic-ultrabasic and granitoid magmas and, on this basis, distinguish (a) differentiated gabbro-tonalite-plagiogranite intrusive series corresponding to accretionary-collisional geodynamic environments, and (b) gabbro-monzonite-granosyenite-potassic granite intrusive series, which were produced when the accretionary-collisional system was affected by a plume.

New data on the composition and age of picritoid and alkaline-basite complexes of the Northern Mongolian segment of the Central Asian fold belt

The new petrochemical and isotope-geochronological characteristics for picritoid and associated alkali-basaltic complexes of the Northern Mongolian Belt (NMB) are given. They are composed in small intrusions, dykes, sills, and volcanic covers with rocks of a higher potassium and titanium content. The studied areas with these units belonged to the western part of the NMB, which stretches sublatitudinally from Uureg Nuur Lake in the west to the south of Trans-Hovsgol Region in the east. By the petrochemical parameters, the complex corresponds to subalkaline basites and lamprophyres, including micaceous plagioperidotites and picrodolerites. The obtained values of Ar-Ar age (342–319 Ma B.P.), together with the known data on other areas in this complex where these rocks are distributed, evidences for a relatively wide (Middle-Late Paleozoic) interval of its origination.

Permian – Triassic Metallogeny

Three mineralization complexes were formed during the Permian – Triassic period in the following order: PGE-Cu-Ni and V-Ti – Fe of magmatic origin; Au – sulfide and Sn – sulfide. The first ore complex including magma-origin Cu-Ni-(Pt) mineralization is related to ultramafic - mafic magmatic differentiation in structures of Song Da, Song Hien rifts (such as Ban Phuc, Ban Khoa, Suoi Cun blocks, etc.) as well as being connected to layered gabbro – peridotite intrusions in folded structures surrounding the Song Chay anticlinoria (such as Nui Chua and Khao Que blocks). Cu-Ni-(PGE)-type mineralization is found in Ta Khoa – and Suoi Cun-type mafic – ultramafic, while Ti-Fe-(V) (mainly) ore type and Cu-Ni-(PGE) are characterized for Nui Chua-type ultramafic intrusion. Radiometric age determined for Ta Khoa ultramafic magmas is 257–270 Ma, for Suoi Cun is 260 Ma and for Nui Chua mafic magmas is 251 Ma; the ages are also applied for the related ore formation, implying that the mineralization of Cu-Ni-(PGE) and Ti-Fe-(V) in geological structures in northern Vietnam was associated with Permian – Triassic tectono-magmatic period. Besides, associated with Cu-Ni-(PGE) ore type is highly potential Ni-Co-As hydrothermal mineralization (discovered in inner contact zones in Ban Phuc block within the Song Da structure). Au-sulfur-type ore complex is widely developed in various structures in northern Vietnam; Representatives of Sn-sulfur-type ore complex are located to the southern margin of Song Hien rift and Lo Gam belt.

Permian – Triassic Pluton – Volcanic Magmatic Associations in the Song Hien Structure, Northeast Vietnam

The Song Hien Basin in northeast Vietnam is linear- shaped, running in the northwest – southeast direction from Vietnam-China border to the easternmost of Lang Son province; with a total length about 200 km the structure divides the Ha Lang belt to the northeast and Phu Ngu belt in the south and southwest. Permian – Triassic magmatism comprises pluton-volcanic felsic and mafic bimodal magmatic associations, including basalt, gabbrodolerite – dolerite (and lherzolite-gabbronorite); rhyodacite – rhyolite and granite. These magmatic associations are spatially, temporally and genetically related. Detailed mineralogical, geochemical and chemically mineralized studies have been conducted intensively on the Song Hien magmatism; among these, the mafic and ultramafic magmas have been described separately while gabbrodolerites were grouped to pluton-volcanic basaltoidic associations. Formation ages of lhrzolite and gabbro-dolerite are 260–262 Ma, while rhyolite – 246 Ma. Geochemical and isotopic data show genetic relationship between basalts and gabbro-dolerite and rhyolite and porphyric granite.

Metallogeny in the Cenozoic

The Cenozoic mineralization in structures associated with the Red River shear zone (RRSZ) in northern Viet Nam includes the following mineralized ore types TR- (U-Th)-Pb-Zn and TR- (U-Th) –Ba-F, Au-Cu in the Song Da rift zone. The origin of the mineralizations is spatially and (possibly) temporally related to the formation of Paleogene potassic mafic – ultramafic and silisic associations. Molybdenum –precious metals (Mo-W-Cu-Au) in the along with Cu-Au mineralization in the Phan Si Pan uplift are related to Paleogene granitoidic magmatism in the Ye Yen Sun complex. Beside, Cu-(Au – REE) mineralizationsin the Phan Si Pan Uplift are described.

An Overview on the Structures, Tectonics and Magmatic Activities in North Vietnam

Reconstruction of paleo-geodynamic framework of Vietnam in the relation to major tectonic events in southeast Asia reveals that Paleozoic structures in Northern Vietnam (from northeastern Song Ma suture zone northward) were developed in the southern and southeastern margins of the Yangtze Block. Northern Vietnam is divided into two major folding systems, based on their positions relative to the Song Hong fault zone.

Paleogene Potassic and Ultra-potassic Volcano-Plutonic Associations in the Song Da Rift

Paleogene potassic and ultra-potassic alkaline volcano-plutonic magmas in the northwestern Song Da structure and in the boundary between this structure and the Phan Si Pan uplift are compositionally diversified, ranging from alkaline mafic (lamproite, absarokite, minette) to alkaline felsic (trachyte and syenite). Geological, mineralogical, geochemical and isotpe characteristics of Ol-Cpx, Cpx-Phl lamproites, absarokite, minette and trachyte and syenite are presented. Lamproite in the northwest Vietnam are compared with low-Ti Mediterranean-type lamproite. Study on the inclusions in clinopyroxene from lamproite showed formation of the potassic and ultra-potassic magmas in the Song Da rift zone is a result of fractional crystallization of melts whose compositions were equivalent to lamproitic magmas, which were subsequently evolved by magma differentiation would lead to form diversified magmas. Mineralogical and geochemical studies of trachyte and syenite showed their genetic relationship. The formation ages of lamproite, trachyte and syenite (42–30 Ma; Ar-Ar and Rb-Sr methods) suggest they are late Paleogene intraplate extension activities in the northwest Vietnam.

Plutonic: Volcanic Associations in the Tu Le Basin and Phan Si Pan Uplift, Northwest Vietnam

Major and trace elements and whole rock Sr–Nd isotope data of Late Permian silicic plutonic and volcanic rocks from the Phan Si Pan–Tu Le region in NW Vietnam were studied in order to establish their petrogenetic relationship with the magmatic rocks of the Song Da zone (NW Vietnam) and the alkaline silicic rocks of the Panxi area of the Emeishan large igneous province (ELIP) in SW China. The granites and rhyolites have geochemical characteristics of anorogenic granites (e.g. high Fe# and high Ga/Al) and are further subdivided based on mineralogy. The Phu Sa Phin and Muong Hum granites contain sodic to sodic–calcic amphiboles and sodic pyroxene, while the Phan Si Pan granite does not. The Phu Sa Phin and Muong Hum granites occasionally show peralkaline to metaluminous compositions, while the Phan Si Pan granites and the Tu Le rhyolites have metaluminous to peraluminous compositions. The chondrite-normalized REE patterns and the primitive mantle-normalized spidergrams (enrichment in high field strength elements) are similar to those of the Song Da silicic rocks. The eNd(t) values range from weakly negative to moderately positive values (−2.2 to +2.2), suggesting the silicic rocks may be assimilated by basement rocks during magma emplacement. Permian magmatic rocks in the Phan Si Pan–Tu Le region are petrologically, geochemically and geochronologically comparable to those of the inner zone of ELIP.

Gabbro and Syenite Intrusions in the Lo Gam Structure, Northeast Vietnam

Lo Gam structure (including Phu Ngu) in northeast Vietnam comprises Permian – Triassic gabbro-peridotite, monzogabbro, high-Al granite and syenite intrusive blocks. The studies have also revealed that gabbro – syenite magmas in folded structures around southeastern margin of the Song Chay anticlinoria form three association types distinguished by their co-existing mineral assemblages, mineralogical compositions and geochemical indexes (titanium content and alkalinity): layered peridotite- gabbro (Nui Chua complex), biotite granite (Phia Bioc complex) and compositionally heterogeneous gabbro-syenite. The uniformity of their geochemical and isotopic compositions indicates, on the one hand, generic relationship of the gabbro and syenite, and geochemically common source origin of the magmas, on the other. Being enriched in large ionic lithophile elements (LILE), rare earth elements (REE) and strongly depleted in Nb, Ta and Zr the gabbroid and syenite magmas reflect typical geochemistry of an above-subduction-zone affiliation. This is the basic feature of Permian – Triassic pluton – volcanic and plutonic magmatic associations in various geological structures in northeastern Vietnam. The formation ages of lherzolite and monzogabbro (Nui Chua complex), gabbro, syenite and biotite granite (Cho Don area) are 251–246 Ma (U-Pb, zircon, SHRIMP, Ar-Ar).

Permian – Triassic Magmatic Activity in the Song Da Structure

Late Permian mafic-ultramafic volcanic and sub-volcanic rocks in the Song Đa Rift include four different associations in terms of low-Ti and high-Ti types. Low-Ti, high-Mg volcanic and sub-volcanic rocks are composed of komatiite, komatiitic basalt and basalt and are divided into three groups according to their petrological and geochemical features. Chemical composition of rocks of the komatiite-basalt association is alkali-low (but rather Na-high), very Ti-low, varying from Al-high komatiite to Al-low basalt. They are characterized by high content of Mg, Al, Ni, Co, Cu and Cr, and low of Ti, Fe, Na, K, P, Rb, Ba, Sr, Nb, Ta, Nd, Hf, Zr and REE. In general, based on geochemical and isotopic characteristics the Song Đa mafic-ultramafic rocks of the komatiite-basalt association may be products of a melt derived from depleted mantle suffering the impact of mantle plume. Digital modeling showed that the initial melt composition was correspondent to komatiitic basalt. Eruption ages of the magmas are 257 ± 24 Ma (by Rb/Sr age dating), and 270 ± 21 Ma (by Re/Os age dating).