Transcript Presentation - Copernicus.org

Mantle xenocrysts of Chompolo field of the
alkaline volcanics, Aldan shield, South Yakutia
E. Nikolenko, N. Tychkov, V. Afanasiev
V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russian Federation
Introduction
Fig. 3. Photomicrographs of garnet (a), Cr-spinel (b) and Mica
(c) xenocrysts in Ogonek pipe; SEM Image of two variety of
K-feldspar in volcanic breccias (d-f)
The Chompolo field of alkali volcanics was discovered in 1957-58 by geologists of the Amakinsky Survey
Group. The breccia dike and pipe structures in the field, composed of wallrock xenoliths, contained typical
kimberlitic minerals (pyrope, Cr spinel, and Cr diopside) and were classified as kimberlites (Shilina and
Zeitlin,1959). More detailed diamond exploration in the area in 1959-1960 was discouraging but later the area
was reconsidered as potentially diamondiferous after twenty four (-1 mm) diamond crystals were found in
1988-1986 (Mudrik, 1988). The Chompolo field belongs to the Aldan lamproite province where such igneous
rocks are widespread and easily accessible Fig. 2 (Tobuk-Khatystyr, Lower & Upper Yakokut fields, Murun and
Lomam plutons) and have been described in numerous publications (Vladykin, et al., 1991; Panina, 1993; Panina
and Vladykin, 1994; Vavilov et al., 1986; Davies et al., 2006). Two events of alkaline magmatism within the
Aldan province, in the Early Proterozoic and Mesozoic (T3-K2) produced, respectively, the Chara-Aldan and
Lena-Aldan subrpovinces (Bogatikov et al., 1991). The igneous rocks were interpreted previously either as
kimberlites (Shilina and Zeitlin, 1959; Utrobin, 1990; Kostrovitsky and Garanin, 1992; Mironyuk, 1998;
Aschepkov et al., 2001), or as lamprophyres and lamproites (Vladykin,1985; Bogatikov, 1991; Panina, 1993;
Panina and Vladykin,1994;Kornilova, 1997; Davies et al., 2006). The rocks of the Chompolo field are hard to
access for sampling and remain poorly studied and little reported. In this study we provide new mineralogical,
petrographic, and chemical data on the Chompolo alkalic igneous rocks
Geological background
Fig. 4. a,c,d - volcanic clasts (autoliths) samples of
Perevalnaya pipe , b -radiaxial feldspar aggregate from
Gornaya pipe (cross nicols); d,e- inequigranular mediumcollisional setting upon a Precambrian cratonic basement (Bogatikov et al, 1991) The Chompolo area belongs to the Central Aldan
to fine-clastic volcanic breccias (cross nicols); f - flow
and lies at the Amga junction between the West Aldan granite-greenstone and the Nimnyr granulite-orthogneiss terranes. The present-day
texture, Sample intrusion 104.
Fig. 5. Contamination index vs the Ilmenite index.The fields of Group I, Group II
kimberlites and lamproites, are shown for comparison (Chalapathi Rao et al., 2003) (a);
TiO2 vs K2O for group I and II kimberlites (after Smith et al, 1985) (b).
The Aldan lamproite province formed in a
superterrane of the Aldan-Stanovik shield
erosion surface exposes a series of closely spaced N—S faults that separate fragments of Archean and Early Proterozoic granite-greenstone and granulite-orthogneiss belts. The
faults accommodate dikes of dolerite and alkaline rocks, kimberlite-like bodies, and large intrusions bearing alkali-earth elements (Fig. 1). The sedments consist of dolomite and
limestone intercalated with Early Cambrian and Late Proterozoic conglomerate. Geological constraints from indicator minerals found in Lower Jurassisediments (Vladimirov et. al.,
1989) place the age of the Chompolo kimberlitesand kimberlite-like rocks at pre-Jurassic, and Bogatikov et al. (1991) suggest a post-Early Triassic age of the kimberlite field. Isotopic
age determinations (Rb-Sr isochron method) of the kimberlite-like body called Intrusion 104 indicate a younger age of 131±4 Ma (Zaitsev and Smelov, 2010).
.
There are ten dike, pipe, and vein structures within the Chompolo field, varying in size from 800
Sampling and analytical techniques
x 25 m (Aldan dike) to 150 x 80 m (Ogonek pipe) or 100 x 40 m (Gornaya pipe) Rock and
mineral amples were collected from six kimberlite-like structures (Aldan, Sputnik, Gornaya,
Ogonek, Perevalnaya, and Kilier-E)during field trips of 2012 and 2013. The analytical work was Samples of the Gornaya, Sputnik, Perevalnaya, and Ogonek structures of the Chompolo field are inequigranular medium- to fine-clastic volcanic breccias with flow texture and a fine-grained groundmass (Fig. 3 and 4). The percentages of
carried out at the Institute of Geology and Mineralogy (Novosibirsk). Mineral chemistry was
wall rock clasts and autoliths range from 20 to 60 %. They are mostly quartzite (micro-quartzite), feldspar and quartz-feldspar, or sedimentary silicic rocks with variable amounts of mica minerals (Fig. 3). The quartz grains are xenogenic
studied on a JEOL JXA-8100 electron microprobe . Rare earths were determined on a Finnigan and have diverse clastic shapes, from isometric to flat (Fig. 4). Clasts of volcanic rocks are less abundant and enclose high percentages of feldspar laths and different amounts of opaque minerals. The volcanic lithoclasts are similar to
MAT ELEMENT high-precision ICP mass spectrometer with a U-5000AT+ ultrasonic nebulizer, the breccia groundmass in phase composition, structure, and texture, and are thus interpreted as autoliths. The fine cryptocrystalline groundmass consists of chlorite, K-feldspar, minor opaque minerals, and mica
using the method of Li metaborate fusion ICP-MS (Nikolaeva et al., 2008). Major oxides were
(muscovite and biotite). The presence of mica in the groundmass may be due to abundant clasts of sedimentary or high-silica igneous rocks. K-feldspar and plagioclase of another variety occur as angular clasts and often contain BaO
measured by X-ray fluorescence (XRF). The samples were examined on a TESCAN MIRA 3 LMU impurity (up to 2.7 wt. %). This variety appears to be more stable against secondary alteration, which may be evidence for its xenogenic origin. All samples contain layered minerals (mica-vermiculate, vermiculate, smectite, kaolinite, and
scanning electron microscope with an EDS Inca Energy 450+ detector.
chlorite) derived from feldspar, mica, etc. Phenocrysts include chromites (Fig. 3), as well as sporadic Sr apatites (up to 2.1 wt.% SrO), rutiles, and zircons, and few garnets (Fig. 3) coated with secondary mineral aggregates (chlorite and
carbonate). XRD analysis has revealed the presence of pyroxene (diopside), amphibole, Sr- and Mn-bearing dolomite, siderite, and calcite.
The rock contains abundant secondary reddish-brownish aggregates as patches or henocrysts,
possibly formed by substitution of mica minerals. . Thus, the igneous bodies in the Chompolo field (except Intrusion 104) are inequigranular volcanic breccias with micro- or crypto-crystalline groundmass of K-feldspar (up to 16.3 wt.%
K2O, up to 3.2 wt.% FeO), chlorite, opaque minerals, melanocratic phenocrysts (garnet, pyroxene, amphibole, Cr spinel, apatite, zircon, mica), and abundant wallrock and basement clasts.
10
Garnet (pyrope) chemistry records the presence of mantle and crustal material. Mantle mineralization prevails in the Aldan dike and the Sputnik, Gornaya, and Ogonek pipes, while crustal and eclogitic signatures are found in the
Perevalnaya and Kilier-E pipes. Mantle garnets lack the common megacryst, wehrlite, and high-temperature lherzolite varieties (Fig. 8). Dunite-harzburgite garnets are always present in minor amounts, without subcalcic high-Cr varieties.
Most of mantle garnets are of lherzolitic paragenesis. Depleted lherzolite garnets are more often of shallow than deep origin. Nevertheless, there are up to 10 mol.% of deep-seated lherzolite garnets with the knorringite endmember. Crustal
garnets are most often of deep granulite origin. Cr spinel (1064 analyses) occurs as 0.2-2.0 mm macrocrysts extracted fromthe heavy fraction of the samples. They are sometimes well faceted octahedrons, most often ith strongly rounded
corners and edges, or segregations of different shapes filling the interstitial space.
Single clinopyroxene
thermobarometer
Conclusions
We infer that the Chompolo alkali volcanic rocks are not kimberlites but rather low-Ti alkaline rocks similar to those in the Aldan province. Unlike
the Aldan lamproite(Tobuk-Khatystyr field, tc.), the Chompolo rocks contain both crustal and mantle minerals. The compositions of pyrope and Cr
Fig.11. Cr-Diopsides from Chompolo pipes in modified Ramsay's
(1992) classification diagram. Solid line - modified boundary between
Gar-lherzolites and Sp-lherzolites.
20
30
40
50
60
Monomineral Cpx thermobarometry (Nimis 2001) was performed on Cr-diopsides with more than 0.5 wt.% Cr2O3 typical of alkaline igneous rocks. When selecting the data for thermobarometry, several compositional groups of diopsides
were excluded. They were, namely (I) non-peridotitic clinopyroxenes; (II) clinopyroxenes from spinel peridotite (Fig.11); (III) Cr diopsides with Al+Cr<Na +K and (IV) those with(Al+Cr) - (Na+K) = from 0 to 0.01. The latter group was excluded
to avoid pressure overestimation known to occur at the (Al+Cr) - (Na+K) difference below 0.01; therefore, correct thermobarometry can be achieved at (Al+Cr) - (Na+K) > 0.01 in Cr diopsides. As a result, only 300 out of more than 1800
analyses remained for the thermobarometry study. . The obtained Cpx thermobarometry results indicate a lithospheric thickness in the sampled Chompolo field no less than 130 km (4.1 GPa) (Fig.12a). The temperature and pressure
points plot along the 35 and 40 mW/m2 geotherms. Igneous bodies in the Chompolo field show alsmot identical geotherm patterns with a characteristic kink in the high-pressure region, which is evidence of interaction between depleted
lithosphere and hotter enriched asthenospheric melts. This interaction is presumed to occur commonly within the lithosphere-asthenosphere transition. A heat flow of 35 mW/m2 is typical of cratonic lithosphere at least 2.5 Ga old and up
to 300 km thick. In the case of samples we studied, heat flow disagrees with the maximum lithospheric depth of 130 km, though the age of the crust (indicative also of the lithospheric age) is older than 2.5 Ga in the Aldan shield.
spinel, as well as thermobarmetry estimates of lithospheric thickness at the time of magmatic activity, indicate that the Chompolo rocks are diamond-barren.
Fig. 12. PT diagrams for mantle
rocks from the Chompolo alkaline
rocks after thermobarometer
(Nimis and Taylor, 2000);
The boundaries of phase transitions:
G/D – graphite/diamond (Kennedy
and Kennedy, 1976)
400
Minerals Chemistry
RFBR 15-05-04885.
The lithospheric thickness
(pressure) was estimated using a
P38 monomineral garnet
barometer (Grutter et al., 2006), for
harzburgitic and lherzolitic
pyropes. The barometry is
originally designed for the cases of
pyrope-chromite coexistence, i.e.,
for Cr-saturated conditions. If this
coexistence is questionable, the use
of a monomineral barometer may
be possible as well, but the results
will
Fig. 10 The
compositional
range of the Cpx
in shown in the
triangular
composition
diagram.
70
80
600
800
1000
1200
1400