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LAMPROPHYRES FROM OSPA OPHIOLITE OF THE EAST SAYAN (RUSSIA)
Semen Kovalev1,2, Sergey Zhmodik1,2, Dmitry Belyanin1,2, Eugenia Airiyants1, Olga Kiseleva1, Yury Kulikov1, Alexey Travin1,2
1 N.V. SOBOLEV INSTITUTE OF GEOLOGY AND MINERALOGY SB RAS, GEOLOGY, NOVOSIBIRSK, RUSSIA, [email protected]
2 NOVOSIBIRSK STATE UNIVERSITY, NOVOSIBIRSK, RUSSIAN FEDERATION
EGU2020: Sharing Geoscience Online, 4-8 May 2020Session GD4.3 Distributed under the Creative Commons
Attribution 4.0 International License
The geographical position
Eastern Sayanmountains are located to the West of Lake Baikal.Eastern Sayanophiolites are a part of the Central-Asian Fold Belt.
Region of work
The geographical position of the Eastern Sayan mountains
The geological position
In the Eastern Sayanmountains (E. Siberia), ophiolite complexes form three extended branches:
1 - Ilchir (MOR ophiolites)2 - Ospa-Khara-Nur(suprasubduction zone (SSZ) and volcanic arc (VA) ophiolites)3 - Shishkhid-Yehe-Shignin(back-arc ophiolites).
Relief map of the south-eastern part of the Eastern Sayan with schematic ophiolite branches position
The history of discovery of the unique rocksLamprophyre (L) dykes or mica peridotites (Shestopalov,
1938) were found in brecciation zone of ophiolites (dunites,harzburgites, serpentinites) of the Ospa-Khara-Nur peridotitecomplex (a part of Ospa-Khara-Nur ophiolite branch) . They formbodies to 1 m thick, and vein-like fragments in intensivelydeformed and altered (serpentinized, tremolitized) ultramaficrocks.
Harzburgite
LamprophyreContact between lamprophyre and harzburgite Lamprophyre sample from Ospa-Khara-Nur ophiolites
Lamprophyre sample from East Sayan. Big amount of mica in gaps between Pl and Amf crystals.
Pl
Phl
Bt
Bt
Amf1 cm
Lamprophyre sample from East Sayan. In that plane there are a lot of medium-grained Pl and Amf, with low amount of mica.
Phl
Phl
Pl
PlAmf
Bt
Amf
1 cm
Amf
Pl
Phl
Lamprophyre sample from East Sayan. Mica in this sample looks like it has filled the voids during magmatic process.
1 cm
Lamprophyre sample from East Sayan. Fine-grained and coarse-grained Pl
Amf
Phl
Pl
Pl
Phl
Phl
Amf
Phl
Phl
1 cm
Phl
Phl
Phl
Phl
Phl
Pl
Pl
Amf
Amf
Xenolith of the host rocks (harzburgite)
Lamprophyre sample from East Sayan. Huge xenoliths of serpentinizedharzburgite with concentric zoning
1 cm
Lamprophyre sample from East Sayan. Huge xenoliths of ultramafic rocks with concentric zoning
Phl
Phl
Phl
Pl
Pl
Pl
Amf
Amf
Amf
Amf
Xenolith of the host rocks (harzburgite)
1 cm
ClassificationDark gray massive porphyric L correspond to the range between ultramafic (UML), alkaline (AL),
Ca alkaline lamprophyre (CAL), and lamproite lamprophyres (LL) according to Rock’s classification(1991) and shows compositional range in MgO-CaO, - Al2O3, - Na2O, - P2O5 diagrams.
CaO/MgO wt.% diagram according to Rock, 1991 (K2O/Al2O3)/(Fe2O3/SiO2) wt.% diagram according to Rock, 1991
Mineral compositionLamprophyre rocks consist of feldspar, phlogopite, orto- and clinopyroxene,
amphibole, with relics of olivine (Fo=45-50, rarely 22-30) and large (up to 1 cm) porphyricphlogopite. In more acid L of CAL type with prevailing hypersthene and fieldspars areassociated by amphiboles metasomatic type (ferro-eckermannite, actinolite, tremolite) andrarely metamorphic glaucophane.
Apt
Bt
Pl
Amf
PlBt
HypAct
Pl
Pl
Bt
Bt
Pl
Act
Results of researches with scanning electron microscope (SEM)
50μm
50μm
Mineral compositionMicas grains from phlogopite to biotite (0.2-1.7% and 2.1-2.8% TiO2) are surrounded by
sericite. Feldspar vary albite to anorthite, and rare grains of orthoclase and Ba-feldspar.Fluorine-apatite (Cl to 0.3%), ilmenite, rutile are common in L but zircon, monazite and Ce-La-epidote are rare. Mineral thermometry range from 1300°C to 950°C for LL then 850°C -560°C and low metamorphic stage.
0,5 mm
Sample scan picture fragment with large amount of mica Mica under the magnification of the microscope
Harker diagrams – TiO2/SiO2
0,0
0,5
1,0
1,5
2,0
2,5
38 40 42 44 46 48 50 52 54 56 58
TIO2
SIO2Harker diagram based on lamprophyres samples – TiO2/SiO2 ratios. The red line indicates a trend
Variation Harker diagrams were built for lamprophyre samples, in order tounderstand the evolution of melt composition.
Harker diagrams – Fe2O3/SiO2
0
2
4
6
8
10
12
14
38 40 42 44 46 48 50 52 54 56 58
Fe2O
3
SiO2Harker diagram based on lamprophyres samples – Fe2O3/SiO2 ratios. The red line indicates a trend
Harker diagrams – MgO/SiO2
0
5
10
15
20
25
30
35
40
45
38 40 42 44 46 48 50 52 54 56 58
MgO
SiO2Harker diagram based on lamprophyres samples – MgO/SiO2 ratios. The red line indicates a trend
Harker diagrams – Al2O3 /SiO2
0
5
10
15
20
25
30
35
38 43 48 53 58
AL 2
O3
SiO2
Harker diagram based on lamprophyres samples – Al2O3 /SiO2 ratios. The red line indicates a trend
Harker diagrams – MnO/SiO2
0,00
0,05
0,10
0,15
0,20
0,25
38 43 48 53 58
MnO
SiO2
Harker diagram based on lamprophyres samples – MnO/SiO2 ratios. The red line indicates a trend
Harker diagrams – CaO/SiO2
0
5
10
15
20
25
38 43 48 53 58
CaO
SiO2
Harker diagram based on lamprophyres samples – CaO/SiO2 ratios. The red line indicates a trend
Harker diagrams – K2O/SiO2
0
1
2
3
4
5
6
38 43 48 53 58
K 2O
SiO2
Harker diagram based on lamprophyres samples – K2O/SiO2 ratios. The red line indicates a trend
Harker diagrams – P2O5/ SiO2
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
38 43 48 53 58
P 2O
5
SiO2Harker diagram based on lamprophyres samples – P2O5/ SiO2 ratios. The red line indicates a trend
Harker diagrams – TiO2/ SiO2
0,0
0,5
1,0
1,5
2,0
2,5
38 43 48 53 58
TiO2
SiO2
Harker diagram based on lamprophyres samples – TiO2/ SiO2 ratios. The red line indicates a trend
REE, HFSE and LILE amountsTRE from L shows inclined REE with flat La-Sm, HFSE troughs but high LILE. The acid
CL reveal Eu peak (Eu*=3,2; (La/Yb)n=9). Spider and REE diagram reveal elevated HFSE,Sr, Pb the same high LILE closer to anorthosites and pegmatiod charnokites. Thissuggests that extremely high temperature ML reacted with acid rocks and producedCa-alkaline L type.
1,00
10,00
100,00
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu«Spidergram» of average compositions for lamprophyre. REE normalized to chondrites, using order and
normalizing values of Nakamura (1974)
REE, HFSE and LILE amounts
«Spidergram» of average compositions for lamprophyre normalized to chondrites, using order and normalizing values of Thompson (1982)
0,10
1,00
10,00
100,00
1000,00
Ba Rb Th K Nb Ta La Ce Sr Pr Nd P Sm Zr Hf Ti Tb Y Tm Yb
Comparison with Rock’s (1991) classification
Two of the founded lamprophyre samples were compared withreference samples from classification of Rock N.M.S (1991).
There are 5 branches of lamprophyric rocks, according to RockN.M.S:
1. Calc-Alkaline (shoshonitic) lamprophyres – (CAL)2. Alkaline lamprophyres – (AL)3. Ultramafic lamprophyres – (UML)4. Kimberlites – (KL)5. Lamproites – (LL)
Comparison of samples with CAL lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)
Comparison with Rock’s (1991) classification
0,010
0,100
1,000
10,000
100,000
1000,000
Sc V Cr
Co Ni
Cu Zn Ga Rb Sr Y Zr Nb Cs
Ba La Ce Pr Nd
Sm Eu Gd Tb Dy
Ho Er Tm Yb Lu Hf
Ta Pb Th U
VS-57 VS-49 CAL
Comparison of samples with AL lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)
Comparison with Rock’s (1991) classification
0,001
0,010
0,100
1,000
10,000
100,000
1000,000
Sc V Cr
Co Ni
Cu Zn Ga Rb Sr Y Zr Nb Cs
Ba La Ce Pr Nd
Sm Eu Gd Tb Dy
Ho Er Tm Yb Lu Hf Ta Pb Th U
VS-57 VS-49 AL
Comparison with Rock’s (1991) classification
Comparison of samples with UML lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)
0,010
0,100
1,000
10,000
100,000
1000,000
Sc V Cr
Co Ni
Cu Zn Ga Rb Sr Y Zr Nb Cs
Ba La Ce Pr Nd
Sm Eu Gd Tb Dy
Ho Er Tm Yb Lu Hf Ta Pb Th U
VS-57 VS-49 UML
Comparison of samples with LL lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)
Comparison with Rock’s (1991) classification
0,010
0,100
1,000
10,000
100,000
1000,000
10000,000Sc V C
rC
o Ni
Cu Zn Ga Rb Sr Y Zr Nb Cs
Ba La Ce Pr Nd
Sm Eu Gd Tb Dy
Ho Er Tm Yb Lu Hf Ta Pb Th U
SAMPLES / LL
VS-57 VS-49 LL
Comparison of samples with KIL lamprophyric rocks. All values normalized to chondrites, using order and normalizing values of Rock (1991)
Comparison with Rock’s (1991) classification
0,010
0,100
1,000
10,000
100,000
1000,000
Sc V Cr
Co Ni
Cu Zn Ga Rb Sr Y Zr Nb Cs
Ba La Ce Pr Nd
Sm Eu Gd Tb Dy
Ho Er Tm Yb Lu Hf Ta Pb Th U
VS-57 VS-49 KIL
Age spectraAge spectra for phlogopites were obtained from lamprophyres by 40Ar/39Ar step heating
method. In sample VS-66-2, spectrum reveal intermediate plateaus of 3 stages (32%, 35%, 33%) ofcumulative 39Ar with ages 950 ± 6 and 976 ± 6 Ma, respectively.
In the spectrum VS-57 a good plateau 902 ± 9 Ma is distinguished (79% of cumulative 39Ar). Mostdiscordant spectrum VS-52 reveals 4 stages of creation. Most likely the age of L formation is - 976 ± 6 Maand corresponds to ocean stage. Most likely, 902 ± 9 corresponds to the age of the intensivedeformation later event in subduction zone. Further deformation suggests the complex tectonic-thermal history.
Age diagram
ConclusionWe suggest that late Proterozoic ophiolites, which refer to oceanic
stage of 1100 Ma, were later incorporated to arc complex with the acidbase. At 980 Ma they were subjected to plume event with the creationof ultramafic lamprophyres due to reaction with the crust and then theywere hybridized with acid rocks to produce calc-alkaline lamprophyres.Late alteration produced series of secondary minerals.
Thus, the ultramafic, alkaline, and alc-alkaline lamprophyres provide us with more information about
the history of ophiolites of the Eastern Sayan.
Thank you for your attention!