Polymetamorphism of Precambrian rocks of the north of Urals

Ulyasheva N.S.

Institute of Geology Komi sc ub ras, Syktyvkar, Russia

nsulasheva@geo.komisc.ru

There are blocks of the Early Proterozoic crust in the north of the Urals represented by deeply metamorphosed amphibolites, schists, and gneisses of the harbeysky complex.

Petrographic and micromineralogical studies of amphibolites and amphibole schists allowed to distinguish several generations of mineral formations using a scanning electron microscope JSM – 6400.

In massive and weakly foliated amphibolites major rock – forming minerals are green amphibole of the pargasite – hastingsite series, oligoclase, and andesine. Quartz grains, clinozoisite and chlorite are rare, the last one often replaces amphibole at the edges.

Garnet (high – calcium pyrope – almandine), clinozoisite, biotite, chlorite, and muscovite are distributed along with blue – green and light green amphiboles of the series pargasite – hastingsite in foliated amphibolites. Albite forms porphyroblasts and also is presented in the form of tabular grains in the intergranular space. Chlorite and muscovite are developed perpendeculary to the schistosity and often replace garnet, indicating their later forming.

Schists were found in local areas and were divided into mainly amphibole, quartz – amphibole, garnet – muscovite – chlorite – clinozoisite – amphibole, and garnet – glaucophane – albite species with the presence of bluish – green, light green amphiboles of the pargasite – hastingsite series, and pale green actinolite.

As a result the minerals parageneses formed under different thermobaric conditions were distinguished. They are pargasite – hastingsite + oligoclase (andesine), pargasite – hastingsite + albite + garnet, pargasite – hastingsite + albite + garnet + clinozoisite, garnet + glaucophane, actinolite + chlorite + clinozoisite, muscovite + chlorite + clinozoisite + quartz.

Parameters Parageneses Порода	Т, 0С			Р, kbar
	Hbl–Pl			Hbl			Hbl–Pl
	1		2	6	7	8	9
	6 кбар	9 кбар
Massive and weakly foliated amphibolites	791	783	639	7,2	7,7	7,5	7–10
	762	737	601	5,9	6,1	6,3
	792	743	595	6,2	6,4	6,6
	644	636	612	5,6	5,8	6
	786	738	525	–	–	–
	699	737	666	9,4	9,7	9,6
	665	688	716	8,7	9	8,9
	660	683	709	–	–	–
Foliated garnet – containing amphibolites	Hbl–Grt						11–12
	3	4	5
	550	590	565	8,7	9,5	9
	575	545	575	8	8,8	8,3
	–	619	643	8,1	8,7	8,4
	580	580	560	9,9	11	10,1
	565	583	515	10,6	11,7	10,8
	530	507	487	–	–	–
	545	537	499	10	11	10,2
	534	530	482	8,9	9,6	9,1
	574	571	525	–	–	–
	550	515	490	9,6	10,6	9,8
	583	582	533	9,9	11,1	10,1
	575	570	620	9,8	10,7	9,9
	552	525	502	–	–	–

Table. Calculated determinations of P – T conditions of the amphibolites forming of the harbeysky complex

Notes. Geotermometers and geobarometers at: 1 – [2], 2 – [4], 3 – [8], 4 – [6], 5 – [5], 6 – [1], 7 – [3], 8 – [7], 9 –[9]. Symbolics: Hbl –amphibole, Pl – plagioclase ,Grt – garnet.

Calculations of thermodynamic parameters of the amphibolites metamorphism were performed using the TPF program developed by A. N. Konilov, A. A. Grafchikov (Institute of Experimental Mineralogy, Chernogolovka), and based on a system of agreed goetermometers and geobarometers, in which the minerals compositions are used in a variety of mineral associations. We applied plagioclase – amphibole [2, 4], garnet – amphibole [5, 6, 8] geotermometers, and also amphibole [1, 3, 7] and plagioclase – amphibole [9] geobarometers (Table).

As a result it was found that temperatures generally range from 600 to 790⁰ in accordance with the pargasite – hastigsite + oligoclase mineral assemblage, in rare cases they are lower. The pressure is 5,5 – 10,0 kbar in the massive and weakly foliated amphibolites. Conditions of the amphibole and plagioclase metamorphism from these rocks correspond to high stages of moderate pressure amphibolite facies.

The garnet – containing parageneses of foliated amphibolites correspond to low stages of metamorphism amphibolite and epidote – amphibolite facies (T 480 – 640⁰) of increased pressures (P 8 – 12 kbar).

The rocks were exposed to low temperature changes in the epidote – amphibolite and green schist facies conditions in local areas.

Thus the harbeysky complex rocks experienced a polimetamorphism in conditions of the moderate pressure high temperature amphibolite epidote – amphibolite facies, and low – temperature changes.

Was supported by program of fundamental research Russian Akademy of Scienses № 12-И-5-2022 and youth project Ural Division of RAS № 11-5-НП-43.

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