Cretaceous Magmatic Rocks Research Articles

Late Cretaceous structural control and Alpine overprint of the high-sulfidation Cu–Au epithermal Chelopech deposit, Srednogorie belt, Bulgaria

The Chelopech epithermal high-sulfidation deposit is located in the Panagyurishte ore district in Bulgaria, which is defined by a NNW alignment of Upper Cretaceous porphyry–Cu and Cu–Au epithermal deposits, and forms part of the Eastern European Banat–Srednogorie belt. Detailed structural mapping and drillcore descriptions have been used to define the structural evolution of the Chelopech deposit from the Late Cretaceous to the present. The Chelopech deposit is characterized by three fault populations including ∼N55, ∼N110, and ∼N155-trending faults, which are also recognized in the entire Panagyurishte district. Mapping and 3-D modeling show that hydrothermal alteration and orebody geometry at Chelopech are controlled by the ∼N55-trending and ∼N110-trending faults. Moreover, the ∼N155-trending faults are parallel to the regional ore deposit alignment of the Panagyurishte ore district. It is concluded that the three fault populations are early features and Late Cretaceous in age, and that they were active during high-sulfidation ore formation at Chelopech. However, the relative fault chronology cannot be deduced anymore due to Late Cretaceous and Tertiary tectonic overprint. Structurally controlled ore formation was followed by Senonian sandstone, limestone, and flysch deposition. The entire Late Cretaceous magmatic and sedimentary rock succession underwent folding, which produced WNW-oriented folds throughout the Panagyurishte district. A subsequent tectonic stage resulted in overthrusting of older rock units along ∼NE-trending reverse faults on the Upper Cretaceous magmatic and sedimentary host rocks of the high-sulfidation epithermal deposit at Chelopech. The three fault populations contemporaneous with ore formation, i.e., the ∼N55-, ∼N110- and ∼N155-trending faults, were reactivated as thrusts or reverse faults, dextral strike–slip faults, and transfer faults, respectively, during this event. Previous studies indicate that the present-day setting is characterized by dextral transtensional strike–slip tectonics. The ∼NE-trending overthrust affecting the Chelopech deposit and the reactivation of the ore-controlling faults are compatible with dextral strike–slip tectonics, but indicate local transpression, thus revealing that the Chelopech deposit might be sited at a transpressive offset within a generally transtensional strike–slip system. The early WNW-trending folds require a roughly NNE–SSW shortening, which is incompatible with the present-day dextral strike–slip tectonic setting and the ∼NE-trending thrust formed during the tectonic overprint of the Chelopech deposit. This reveals a rotation of the principal stress axes after Late Cretaceous high-sulfidation ore formation and post-ore deposition of sedimentary rocks. The nature of the sedimentary rocks interlayered and immediately covering the Upper Cretaceous magmatic rocks hosting the Chelopech deposit indicates sedimentation and associated volcanism in an extensional setting immediately before ore formation. It is concluded that the Chelopech deposit was formed when the tectonic setting changed from extensional during Late Cretaceous basin sedimentation and magmatism, to compressional producing WNW-trending folds under a roughly NNE–SSW compression, possibly in a sinistral strike–slip system. Thus, like other world-class, high-sulfidation epithermal deposits, the Chelopech deposit was formed at the end of an extensional period or during a transient period of stress relaxation, which are particularly favorable tectonic settings for the formation of high-sulfidation epithermal deposits. The exceptional preservation of the Upper Cretaceous Chelopech epithermal deposit is explained by the combined deposition of a thick Senonian sedimentary sequence on top of the Upper Cretaceous magmatic host rocks of the deposit, and the later overthrust of older rock units on top of the deposit. Our study at Chelopech supports previous studies stating that post-ore basin sedimentation and tectonic processes provide the favorable environment to preserve old epithermal deposits from erosion. The tectonic evolution of the Chelopech deposit is similar to that of the entire Panagyurishte ore district. This coherence of the magmatic, hydrothermal, and tectonic events from north to south suggests that the ore deposits of the entire Panagyurishte ore district were formed in a similar tectonic environment.

Tectonics and sedimentation in convergent margin basins: an example from the Tertiary Elazığ basin, Eastern Turkey

The Tertiary units of the Elazığ basin consist of Lower Paleocene continental deposits, Upper Paleocene–Lower Miocene marine deposits and Pliocene–Quaternary continental deposits. In the Early Paleocene, alluvial fan deposits developed at the front of a thrust related to southward emplacement of Permo-Triassic metamorphic rocks onto Upper Cretaceous magmatic rocks. Marine conditions developed in the basin as a result of an extensional regime that started during the Late Paleocene and continued until Early Miocene time. In the Elazığ basin, Late Paleocene–Early Eocene time is represented by shallow-marine limestones while the Middle–Late Eocene sequence includes clastic and calcareous rocks of shelf, slope, slope apron and basin plain environments. The Tethys sea became shallower and retreated to the north during the Oligocene. Shallow-marine carbonate and clastics were deposited in the basin from Oligocene to Early Miocene time. Fluvial and lacustrine deposits accompanied by Early Pliocene volcanism filled E–W trending intermontane basins related to N–S tension beginning in the Early Pliocene. Alluvial fan and fluvial deposits accumulated during the Plio-Quaternary and were related to ENE–WSW aligned folds and thrusts.

Facies and evaporite genesis of the Kuşcular Formation (Lower Paleocene) saline playa complex, Eastern Turkey

The Kuşcular Formation (Lower Paleocene), located in Eastern Turkey, is an alluvial and playa foreland basin succession deposited in a semi-arid climate. The playa deposits of the Kuşcular Formation, composed of interbedded green sandstone–siltstone–mudrock, gypsiferous sandstone, red-green gypsiferous mudrock, and banded nodular secondary gypsum, are found among the distal alluvial fan deposits. Interbedded green sandstone–siltstone–mudrock with desiccation cracks are interpreted as dry mudflat–sandflat deposits. The banded nodular secondary gypsum and gypsiferous mudrocks, which contain desiccation cracks, are interpreted as a saline playa and saline mudflat. Distal alluvial fan deposits are represented by stratified conglomerate and stratified pebbly sandstone. The sulfur in the evaporites was very likely derived from the oxidation of sulfides in Upper Cretaceous magmatic rocks comprising the basement of the basin. Soft sediment deformation structures (slump folds), which indicate seismically active areas, were formed within the playa and dry mudflad–sandflat complex. The facies aspects of playa deposits would suggest that the climate in Turkey was semi-arid during the Early Paleocene.

The Karnataka Late Cretaceous Dykes as products of the Marion Hot Spot at the Madagascar ‐ India Breakup Event: Evidence from 40Ar‐39Ar geochronology and geochemistry

Two late Cretaceous mafic dykes with an ENE strike that is orthogonal to the west coast of India and located nearly 200 km inland around Huliyardurga, Karnataka state, yield 40Ar‐39Ar plateau ages of 90.0±1.0 and 87.5±0.9 Ma. These Fe‐Ti‐enriched tholeiites are essentially co‐eval with at least four other igneous suites widely scattered in southern India, namely; the south and north Kerala dykes, the Agali‐Anaikatti dykes of central Kerala‐Tamil Nadu and lavas of St Mary islands off the west coast of India. The Karnataka Cretaceous dykes are also co‐eval and compositionally very similar to the Fe‐Ti‐enriched tholeiitic lavas and dykes around Mananjary, a major phase of Late Cretaceous magmatism along the eastern rifted margin of Madagascar which are believed to be products of the Marion hot spot that extruded at ∼88 Ma, synchronous with the India‐Madagascar break up event. The age and compositional similarities between these Late Cretaceous magmatic rocks from India and Madagascar constitute a clear evidence for extension of the thermal manifestations deep into the Indian peninsula.

The paleomagnetism of the Upper Cretaceous magmatic rocks in the Banat area of South Carpathians: tectonic implications

Forty-four sites were sampled in Upper Cretaceous magmatic rocks in the western part of the Southern Carpathians (Banat area) for a paleomagnetic study. After detailed AF demagnetization, only 10 sites yielded primary remanent magnetizations, the directions of which are similar to those identified in Senonian magmatic rocks from the Apuseni Mountains and the Haţeg and Rusca Montană Basins. The mean paleomagnetic pole for these three areas is compared with those from coeval formations of stable Europe and stable Africa. This comparison shows a large clockwise rotation, both with respect to Africa and Europe, and a northward transport to reach their present-day position during the Tertiary. The relations with adjacent zones of the Tethys mobile belt are also discussed.

Tectonic implications of paleomagnetic research into Upper Cretaceous magmatic rocks in the Apuseni Mountains, Romania

The paleomagnetism of Upper Cretaceous magmatic rocks from 47 collecting sites (172 samples, 692 specimens) in the Apuseni Mountains was studied. After AF cleaning, characteristic magnetizations were identified for various collecting areas in the study zone, which defined a few spatial and temporal units for which paleomagnetic poles could be derived statistically. At 21 sampling sites the paleomagnetic directions showed a high level of intrasite and intersite consistency, with a mean direction of I f = −38° and D f = −100°, with α 95 = 6°. The paleomagnetic results show that to reach their present-day position the Apuseni Mountains moved to the north, around 14° with respect to Europe, or around 25° with respect to the geographic poles, between the Campanian and, probably, Late Miocene, while a clockwise rotation, of around 80°, was taking place.

Paleomagnetic studies on sedimentary Jurassic rocks from southern Bulgaria

Paleomagnetic investigations were performed on Jurassic sediments sampled in the mobile area of Bulgaria comprising Srednogorie, Kreiste and Stranja. The main carriers of magnetic properties of rocks studied are pigmentary haematite and goethite of post-sedimentary origin. The characteristic component of natural remanence (CARM), isolated by means of analysis of demagnetization procedure is secondary, and was acquired after the main tectonic event that took place in this area during the Upper Cretaceous. The CARM directions before tectonic correction are close to the results for Upper Cretaceous magmatic rocks from Srednogorie. The mean CARM direction and the corresponding polar position obtained for the whole region studied are compared with reference data for the Eurasian Platform. The difference between the reference and our data implies anticlockwise rotation of southern Bulgaria relative to Eurasia by ∼ 10–20° and a northward tilt of the region under study by ∼ 10–15° after acquisition of CARM.

A palaeomagnetic approach to the origin of the Black Sea

Available palaeomagnetic data for the circum-Black Sea region are reviewed with the object of evaluating the geodynamical evolution of the Black Sea. Jurassic data from the south of the Stara planina-Forebalkan zone (south of the Moesian platform) in Bulgaria, which was then part of the east European platform, place the region at about 39° latitude; while the Pontides, the northernmost tectonic unit of Turkey, were at a lower latitude (10–15°). Palaeomagnetic investigations on Upper Cretaceous magmatic rocks sampled in different part of the Srednogorie tectonic unit (southern boundary of the Stara planina-Forebalkan) indicate that the region was at 39° palaeolatitude during the Upper Cretaceous, while the Pontides had moved northward to 23° palaeolatitude since Early Cretaceous time. The Great Caucasus domain of the USSR was welded to Eurasia by Coniacian time and had a palaeolatitude of 36°. Calculated palaeopole positions for the above-mentioned geological times for the Pontides fit well with the African palaepoles but largely deviate from the Eurasian ones. Taken together, these observations indicate that the Pontides were not part of Eurasia; instead, they were separated from the east European platform by the early Mesozoic Tethys. This conclusion does not favour the rifted ‘back-arc’ hypothesis for the formation of the Black Sea during Cretaceous time, as suggested by some researchers (Adamia et al. 1977, Hsü et al. 1977, Sengör & Yilmaz 1981).

Petrology of the Cretaceous magmatic rocks from Meratus Range, southeast Kalimantan

45 Crataceous magmatic rocks from the Meratus Range, southernmost Kalimantan, have been studied by petrographic, chemical (major- and trace-elements) and mineralogical (microprobe) analyses. They belong to the Alino Formation (Upper Aptian to Cenomanian), the Peridotitic Nappe overthrusted on the Alino Formation, and the Upper Turonian to Senonian Manunggul Formation, uncomformably overlying the Peridotitic Nappe. All the magmatic rocks studied have suffered hydrothermal/greenschist facies alteration; therefore petrologic studies are concerned with “immobile” or “less mobile” major and trace elements and with compositions of relicts of the primary minerals (Ca-clinopyroxene phenocrysts). With the exception of the Riam Andungan plagiogranites, which have been considered as original member of the Peridotitic Nappe, all the volcanic and plutonic rocks of the Manunggul Formation as well as the plutonic rocks intruding the Peridotitic Nappe are definitely of island-arc calc-alkaline affinity. This is also true for most volcanic rocks of the Alino Formation, with the exception of analcite-bearing basalts which prove to belong to an alkaline (K-rich, silica-undersaturated) island arc association. Thus a subduction-related tectonic environment is proposed for the Middle and Late Cretaceous evolution of the Meratus Range both before (Alino Formation) and after (Manunggul Formation) the obduction of the Peridotitic Nappe.